I have this wierd problem with my pc:

We bought a pc with window xp pre installed, with 128 RAM. I downgraded it to windows 2000 since it was for some reason slow. It is a p4 1.8. I'm sure that's weird, but it is slow.
Anyway it worked fine, after buying additional ram I decided to upgrade to windows xp Professional.
After a few reboots the hard drive fails.
So I repair via the installation screen.
It was okay after a few reboots, then it fails.

Here's a list of what I tried to do after the above:

Erased the hard drive, reinstalled a fresh installation of windows Xp prof. < Failure
Replace RAM for testing one after the other < failure

Afterward wards I erased the hard drive then reinstalled window 2000 < Good as new
NOW WHY?

It works in safemode but just fails in normal boot

Please help