Comp reboots repeatedly on startup

[sizk]

Usually it (an old P4 2.8 Ghz with a P4P800 mobo) keeps rebooting for a while on startup then finally gets to start for good. Sometimes - more rarely - it doesn't boot at all, sometimes it reboots on Windows Start, sometimes before.

At first I thought there was something wrong with Win2K which had been installed for many years without much maintenance so I installed XP but the problem remained.
I thought it could be the power supply but once Windows started it's working fine, doesn't reboot or crash so it's probably not the power supply.
Then I don't know much about hardware failure so I'm out of clue.

Thanks for any help.

 

[Dystopia]

Messed up hard drive? Like once it'll start spinning, it'll work, but at first it kinda stuck? Other than that, I can't help ya.

 

[sizk]

That's something to consider, I'll do a test with another hard drive later cause it's not my own comp.
In the meantime any other suggestions welcome