System crash everytime i login after a few minutes

Could you have a look at end of files /var/log/syslog ?

I tried but I got the following messages

nicola@nicola-desktop:~$ files /var/log/syslog
No command 'files' found, did you mean:
 Command 'file' from package 'file' (main)
files: command not found
nicola@nicola-desktop:~$

I attach the last lines

Jan 31 15:46:34 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 4 of 5 (IP6 Configure Get) scheduled...
Jan 31 15:46:34 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 3 of 5 (IP Configure Start) complete.
Jan 31 15:46:34 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 4 of 5 (IP6 Configure Get) started...
Jan 31 15:46:34 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 4 of 5 (IP6 Configure Get) complete.
Jan 31 15:46:34 nicola-desktop NetworkManager: <info> DHCP: device eth0 state changed (null) -> preinit
Jan 31 15:46:34 nicola-desktop dhclient: Listening on LPF/eth0/00:1a:a0:2f:e5:18
Jan 31 15:46:34 nicola-desktop dhclient: Sending on LPF/eth0/00:1a:a0:2f:e5:18
Jan 31 15:46:34 nicola-desktop dhclient: Sending on Socket/fallback
Jan 31 15:46:34 nicola-desktop anacron[1716]: Anacron 2.3 started on 2010-01-31
Jan 31 15:46:34 nicola-desktop anacron[1716]: Normal exit (0 jobs run)
Jan 31 15:46:35 nicola-desktop avahi-daemon[880]: Registering new address record for fe80::21a:a0ff:fe2f:e518 on eth0.*.
Jan 31 15:46:36 nicola-desktop dhclient: DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 6
Jan 31 15:46:36 nicola-desktop dhclient: DHCPOFFER of 192.168.123.133 from 192.168.123.254
Jan 31 15:46:36 nicola-desktop dhclient: DHCPREQUEST of 192.168.123.133 on eth0 to 255.255.255.255 port 67
Jan 31 15:46:36 nicola-desktop dhclient: DHCPACK of 192.168.123.133 from 192.168.123.254
Jan 31 15:46:36 nicola-desktop dhclient: bound to 192.168.123.133 -- renewal in 34677 seconds.
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> DHCP: device eth0 state changed preinit -> bound
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 4 of 5 (IP4 Configure Get) scheduled...
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 4 of 5 (IP4 Configure Get) started...
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> address 192.168.123.133
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> prefix 24 (255.255.255.0)
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> gateway 192.168.123.254
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> nameserver '89.19.64.36'
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> nameserver '89.19.64.164'
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 5 of 5 (IP Configure Commit) scheduled...
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 4 of 5 (IP4 Configure Get) complete.
Jan 31 15:46:36 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 5 of 5 (IP Configure Commit) started...
Jan 31 15:46:36 nicola-desktop avahi-daemon[880]: Joining mDNS multicast group on interface eth0.IPv4 with address 192.168.123.133.
Jan 31 15:46:36 nicola-desktop avahi-daemon[880]: New relevant interface eth0.IPv4 for mDNS.
Jan 31 15:46:36 nicola-desktop avahi-daemon[880]: Registering new address record for 192.168.123.133 on eth0.IPv4.
Jan 31 15:46:37 nicola-desktop NetworkManager: <info> (eth0): device state change: 7 -> 8 (reason 0)
Jan 31 15:46:37 nicola-desktop NetworkManager: <info> Policy set 'Auto eth0' (eth0) as default for routing and DNS.
Jan 31 15:46:37 nicola-desktop NetworkManager: <info> Activation (eth0) successful, device activated.
Jan 31 15:46:37 nicola-desktop NetworkManager: <info> Activation (eth0) Stage 5 of 5 (IP Configure Commit) complete.
Jan 31 15:46:36 nicola-desktop ntpdate[1798]: step time server 91.189.94.4 offset -1.013550 sec
Jan 31 15:46:43 nicola-desktop kernel: [ 30.268013] eth0: no IPv6 routers present
Jan 31 15:47:32 nicola-desktop kernel: [ 79.500020] Clocksource tsc unstable (delta = -222204261 ns)

In syslog, you have message "Clocksource tsc unstable".
It seems to be related to acpi and clock.

Solution proposed in forum is to add in "/boot/groob/menu.lst" at end "clocksource=hpet" or "clocksource=hpet acpi=off noacpi"
like in the line below
kernel /boot/vmlinuz-2.6.24-19-generic root=UUID=a4646 ro quiet splash xforcevesa clocksource=hpet acpi=off noacpi

To avoid crashing, you can use the Installation CD-ROM as a Live CD, mount the root partition and edit the file under the new directory of mount point.

I attach my menu.lst. Could you please advise where I should add 'clocksource=hpet acpi=off noacp'

title Ubuntu 9.10, kernel 2.6.31-18-generic
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-18-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro quiet splash
initrd /boot/initrd.img-2.6.31-18-generic
quiet

title Ubuntu 9.10, kernel 2.6.31-18-generic (recovery mode)
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-18-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro single
initrd /boot/initrd.img-2.6.31-18-generic

title Ubuntu 9.10, kernel 2.6.31-16-generic
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-16-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro quiet splash
initrd /boot/initrd.img-2.6.31-16-generic
quiet

title Ubuntu 9.10, kernel 2.6.31-16-generic (recovery mode)
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-16-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro single
initrd /boot/initrd.img-2.6.31-16-generic

title Ubuntu 9.10, kernel 2.6.31-15-generic
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-15-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro quiet splash
initrd /boot/initrd.img-2.6.31-15-generic
quiet

title Ubuntu 9.10, kernel 2.6.31-15-generic (recovery mode)
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-15-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro single
initrd /boot/initrd.img-2.6.31-15-generic

title Ubuntu 9.10, memtest86+
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/memtest86+.bin
quiet

### END DEBIAN AUTOMAGIC KERNELS LIST

# This is a divider, added to separate the menu items below from the Debian
# ones.
title Other operating systems:
root

# This entry automatically added by the Debian installer for a non-linux OS
# on /dev/sda1
title Dell Utility Partition
root (hd0,0)
savedefault
makeactive
chainloader +1

# This entry automatically added by the Debian installer for a non-linux OS
# on /dev/sda3
title Windows Vista (loader)
rootnoverify (hd0,2)
savedefault
makeactive
chainloader +1

Can I remove from the list the recovery modes?

Also my CD is Ubuntu 9.04 Desktop i386 but I upgraded to Ubuntu Karmic. Can I still use the CD and can you give me more instruction as to how to mount the root partition as I have never done that before?

title Ubuntu 9.10, kernel 2.6.31-18-generic
uuid 3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf
kernel /boot/vmlinuz-2.6.31-18-generic root=UUID=3b6c2b2f-bd55-45f0-8b4f-cbd1b93ec2bf ro quiet splash clocksource=hpet acpi=off noacpi
initrd /boot/initrd.img-2.6.31-18-generic
quiet

For mounting System->Administration->Disk Utility Palimpseste
Select the partition, click at icon upper left (disk wirh help label mount).

After we will see for recovery mode removing.
And if it works...

I cannot save menu.lst as I do not have permission. Ho do I opne the file as root?

D'abord faire une copie de sauvegarde.
"sudo vi menu.lst", puis entrer le mot de passe administrateur
Remplace "vi" par le nom de ton éditeur favori (par exemple gedit).

I added 'clocksource=hpet acpi=off noacpi' to menu.lst and saved it but I still don't know how to mount the root partition. Which partition should I select in System->Administration->Disk Utility Palimpseste?

Sorry for the french, but I switch from english to french launchpad!
If you added "clocksource=hpet acpi=off noacpi", it means you used the root partition.
So I presumed you use recovery mode (and not a Live CD), which is also OK.
Now, you have to check that solution solves your problem.

It did not solve the problem as the system carshed again while I was surfing the web with Firefox.

Summary
   1) New installation from Ubuntu 9.04, followed by a migration to Karmic.
   2)The whole desktop is displayed, and after Ubuntu "crash".
   3) Last dmesg line is: "Clocksource tsc unstable (delta = -222204261 ns)"
Are previous data right ?

Do you have data to recover if a fresh installation from Ubuntu 9.10 CD is required ?
   following french forum, could be a migration problem from 9.04 to 9.10
The crash is a freeze or the computer reboot (or shutdown) ?
Can you confirm you have no WIFI ?
When it freezes, try Ctrl+Alt+F8 then Ctrl+alt+F1. What happen ?

A proposed solution is (force repairing of some kernel components):

sudo -s
aptitude update && aptitude upgrade
aptitude dist-upgrade
aptitude install build-essential linux-restricted-modules-$(uname -r)
aptitude install linux-headers-$(uname -r)
aptitude autoclean && update-pciids && update-usbids
reboot

Another one is to install kernel 2.6.32 (linux-image-2.6.32-020632-generic_2.6.32-020632_i386.deb).

Personnaly, if you have no data to recover, I advise to reintall from 9.10 CD, which will provide a healthier base of work.

The system freezes and everything stops working so the only way to shut down the PC is to use the power button. The same problem occured in Ubuntu 9.04 before I upgraded to Karmic and affected Windows as well. I tested the memory and found no problems and afterwards the PC got back to work.
What is WIFI? I do not think I have WIFI anyway.
I never tried Ctrl+Alt+F8 or Ctrl+alt+F1 but I doubt they work.
Windows works fine whereas Ubuntu Karmic keeps freezing evry now and then with no apparent reason.

Will I try the Kernel repair you suggested?

Do you have data to recover ?

Does the problem also occur with Live CD ?

"I never tried Ctrl+Alt+F8 or Ctrl+alt+F1 but I doubt they work." => Don't presume, try! If it works, problem lies in graphic card driver.
If you can't switch to a terminal, which use the whole screen, try to repair the kernel.

I never tried the Live CD.

Next time I will try Ctrl+Alt+F8 or Ctrl+alt+F1 and will let you know.

I do not have much data to recover. Do you suggest I reinstall Ubuntu Karmic from scratch?

System froze again but Ctrl+Alt+F8 or Ctrl+alt+F1 did not work.

Will I try the Kernel fix you gave me?

I don't know. Try it, I think there are few risk.

I think this a bug and should be resolved. I had to give up Ubuntu altogether for this problem.