The first thing I would do is go into the BIOS and turn off all power saving options, then go into XP and do the same.

I have come across this before, and it is usually the hibernate/sleep option doesn't work with some elements of the hardware.

It is a process of elimination. If what I have suggested does not work then it sounds like a hardware or malware problem as already suggested.

It would probably do no harm to make sure that your drivers are up to date as well?