Recurring Restart Problem

Hey Guys,
For a long time now I have had a problem with my computer. Randomly when I am using it (or if I am not) it restarts. I don't mean "windows is shutting down" restarts, it acts like it has lost power and turns backo on.

Over the last few months I have tried many different things to fix the problem:
- I tried seeing if a borrowed poer supply would fix it
- I Got a new hard drive on which a installed a fresh version of XP Sp2 (Up from Win 2K)
- I have completed many spyware and virus checks.
- I have run my computer of different power sockets (also this problem occurs at LANs at other peoples houses)
- I updated the BIOS (I think... I am very new at this)
- I even took it into a proffesional computer mantinance shop, to no avail

I am really stuck for ideas. This problem sometimes occers often (every few minutes), some times hours, sometimes days. It is very intrusive. If anybody can suggest anyhing I would be very greatful.

I am using XP SP2
I have 1 Gig Ram
Pentium 4 3.2
250 SATA (now)
Gigabite Motherboard

Thanks in advance,
Erin