XP rebooting at logon

Ok i have really wierd problem...last night i was fixing my GF's (trying to) hard drive and then i couldnt fix it so i plugged my old hard drive back in,booted up and played til about 3am.

Now i wake up this morning turn on my computer, logon and about 4seconds in, it just reboots.

I have returned overclock to factory settings and its still doing it(that eliminates the overclock.)

Its obviously windows because im in safe mode right now. It seems very laggy though in safe mode...i dont know if thats normal or not.

What could be the problem ?

EDIT: I just ran spybot and it didnt find anything.

Could bea a number of things. Not just windows though. It could be bad hardware like ram oor HDD. As windows tries to load a file from that sector of either of those items it runs into trouble and restarts. Could also be windows like you said, some sort of system fiel missing. Though uselly that brings up an error message. I would try a repair install if you have the disk. If not luck there I would start to lean towards hardware issue.

ok ill do a repair.

Sounds to me like a memory problem.

shall i run memtest86 and see ?

did you ever have your gf's harddrive and your hd in the computer at the same time ?