I've seen this problem before and it's usually caused by one of two things:
1) Bad OS installation - If XP hasn't been reinstalled in the past 6 years, there is a decent likelihood that something has become corrupted. Try to boot into safe mode by pressing F8 right after the BIOS screen and see if you can get in to the OS for more diagnostics.
2) Hardware problem - Two possibilities here. First reseat *every* connection in the system as small amounts of corrosion on the connections can affect things after long periods of time. If this doesn't fix the issue, it is possible that your RAM has degraded and is no longer stable. Try running with less memory and use different sticks on each attempt. If this solves the problem, you can just run with less memory or purchase replacement chips.
Good luck.