ubuntu 8.04 dos'nt always boot

ssuming drive cache: write through. Boot Problem
I have recently installed Ubuntu 8.04 desktop i 386 on my computer which has Vista Home Premium on the C drive and I installed Ubuntu on the D drive. When I start the computer it gives me a option of starting Vista or Ubuntu. Vista starts with no problem however Ubuntu sometimes boots OK and other times the progress bar stops and a new screen comes up with the following message

BusyBox V1.1.3 (Debian 1.1.3-5 ubuntu12) Built-in Shell (ash)
Enter 'help' for a list of built-in commands.

(initramfs) [57.252032] usb 6-1 : device descriptor read/64, error-110
[72.455690] usb 6-1 : device descriptor read/64, error-110
[87.771248] usb 6-1 : device descriptor read/64, error-110
[102.974897] usb 6-1 : device descriptor read/64, error-110
[113.590272] usb 6-1 ; device not accepting address 4, error-110
[124.101732] usb 6.1 ; device not accepting address 5, error-110

At this point if I restart the computer sometimes it will boot Ubuntu OK and other times the progress bar stops and it returns to the screen shown above. Or to this screen
BusyBox V1.1.3 (Debian 1.1.3-5 ubuntu12) Built-in Shell (ash)
Enter 'help' for a list of built-in commands.

(initramfs) [56.879945] sd 8:0:0:0: [sdc] Assuming drive cache: write through.
  [56.881188] sd 8:0:0:0: [sdc]Assuming drive cache: write through.
  [56.891549] sd 10:0:0:0: [sdf] Assuming drive cache: write through.
  [56.893422] sd 10:0:0:0: [sdf] Assuming drive cache: write through.

Sometimes if it fails to boot i use the safe mode and it runs through a long list of checks and then gives me a option of normal boot, if I select this it boots OK. Sometimes in safe mode it goes partway through the checks and stops and dosen't boot.

I downloaded Ubuntu and burned it to a CD and installed using the installer on the CD and Ubuntu seemed to install OK. Perhaps I should uninstall and reinstall Ubuntu? or can I reinstall over the existing installation?
Thanks for any help you can give me.
Al