My computer restarts itself

Sounds to me like XP is crashing. XP will reboot instead of throwing a BSOD. If you have any time at all before it crashes, do this:

Right click My Computer > Properties > Advanced
Click Settings button for Startup and Recovery
Uncheck Automatically Restart

Then when XP crashes, you'll get the good ol' BSOD. At least then you should be able to get a vague idea of what's going on.

If it's not Windows crashing then check for hardware problems. Since you're working off a fresh install, I'd say loose cables or overheating. Reseat all your expansion cards and reconnect all your cables. Make sure your CPU and PSU fans are working right (run the PC with the side panel off so you can see everything).

Not at all, eh. Another way to possibly test Windows vs Hardware is to go into your CMOS Setup while you're booting up. If you can sit in CMOS setup for as long as you want without a reboot, it's probably Windows-related. If not, then you've eliminated Windows as the cause and should start looking at hardware.