Well, it looks like it's a power supply or motherboard problem. Try to make the computer read a disc, when it restarts enter to the BIOS and check the heat monitor inmediately, if the temperature is higher than normal maybe the power supply is the trouble, try using another to see if it keeps happening.
I am having a kinda simmilar problem with my HDD which randomly loses power and you hear it now and again firing back up although it doesn't affect my xp running and since its only a temporary 40gb drive i'm not worried about changing PSU for the moment but will do soon. I would suggest a change to the power supply as it looks like that could be the problem, if you can try bigger Power supply or another one first just to be sure its that and if its not that then your motherboard is most likey the problem.
Yes, it's called power saving - check both XP and the BIOS for hard drive spindown settings. And no, that's not the same problem as karstens.
i've bought a new case+power (600w this time!)... i'm finishing reinstalling all the parts. later today i'll test if everything works fine! i'll keep you all informed. thanks for the help!
Good to hear it ^_^ It was the most probable cause, something similar happened to me before, but after the power source just died
Power supply problems can cause anything. BSoDs, PCI cards crapping out, instability, and whatever else.