Unpredictable freezes in WinXP

I have a 866 mhz pentium 3, 386 mb SDRAM, 32 mb video card and a 30 gig hard drive, running under Windows XP Professional.

SCENARIO:
My PC Unpredictably freezes, either on startup or 5 hours after startup, the computer completley freezes reulting in me restarting – when i do start my computer up, theres a high chance that it will freeze at the Windows XP Pro loading screen. this will happen a number of times so i have to repeatedly restart until it does startup, but then it might freeze again when windows has loaded. i have foematted numerous times to rectify the problem but it still happens. i tried replacing my SDRAM with a freind of mine suspecting that this was the problem but it still happen. I think it is a hardware fault…Can ANYONE please shed some light?