GeForce 8400 GS : 10.04 won't boot, die after login screen

You probably have an issue with hardware management, and probably the graphic card.
And don't know how to debug Wubi.
For UNetBootIn, boot on desktop and open a terminal (Applications->Accessories->Terminal)
    cd /boot/grub
    sudo gedit grub.cfg
On first linux menu, remove string "quiet splash", and save file. (it's not the clean way of doing, but the faster)
Reboot on portable.
Now, instead of seeing "five dots" screen, you see messages of progression of initialisation process.
Which is the last message ?
Also, what is your graphic processor ?

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/372671

sudo is not to be used for gedit, use gksudo.

If you hold shift at boot, you can choose recovery mode, the select root.You can now run

nano /etc/default/grub

change:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
to:
GRUB_CMDLINE_LINUX_DEFAULT=""

Press CTRL+X
Press Y
Press ENTER

then run:

update-grub; shutdown -r now

This will apply the settings and reboot the OS. You will now be able to see the boot rather than the stupid boot splash hiding all the useful information.

I've tried both tips but my PC still hangs at start-up. The only difference now is I don't see the red and white flashing dots anymore. The command "shutdown -r" doesn't restart my PC. It just keeps asking me to check the help page. I think somethings wrong with my PC because the portable flash I made with UNetbootin works on other computers.

Which is the last message ?
Also, what is your graphic processor ?

the command is:

shutdown -r now
NOT
shutdown -r

Please don't run partial commands that people give, you can damage your system.

Now you have removed the spash, what is onscreen when it hangs?

I have NVIDIA GeForce 8400 GS. I just updated the driver.

After "shutdown -r now", the system rebooted and the screen with the dots was gone. It then showed the login screen but went back immediately to a black screen showing this then hanged again:

init: ureadahead-other main process (345) terminated with status 4
fsck from util-linux-ng 2.17.2
/dev/loop0: Superblock last mount time is in the future.
              (by less than a day, probably due to the hardware clock being incorrectly set) FIXED.
 * Starting AppArmor profiles
Skipping profile in /etc/apparmor.d/disable: usr.bin.firefox
                                                                                                                                                  [OK]
 * Setting sensors limits

I'm a little confused about error message, and the bug I linked to your question.
It seems to be a badly formed "Warning" message, with any relationship with actual problem.
I think graphic issue is the most probable.
Could you reopen the question, to get attention of a graphic expert (what I'm not) ?

Thanks delance, that solved my question.

I removed my magicJack and Ubuntu loaded easily without any more problems. When you asked if I had a removable disk connected, I remembered my magicJack. It probably didn't have any other problems in the first place and I didn't even have to get rid of the splash screen.

Thanks!