boot up problems
First off, let me say that i don't know if this is an Ubuntu problem or not, but i thought i would start here.
I have a dual boot machine win xp home/ unbuntu 8.04.
I have had on a couple occasions where i can not boot up into ubuntu. It will just lock up during the boot process. Sometimes, it doesn't even get to the GRUB(i think) menu to select op sys.
I have noticed that when this happens(mostly after hours of trying to get ubuntu to boot) that if i boot to xp, then restart, ubuntu will load.
Re: boot up problems
if it locks up _before_ Grub is even starting, it looks like a hardware defect.
does your bios perform a POST (power-on self test) every time it boots? does it state anything useful?
Does it really lock up? Did you try to let it run a while?