I had the exact same issue when my broadband cable modem was hooked up via USB. The modem wasn't 100% compatible with WinXP, and it caused random restarts.
Go to Control Panel -> System -> Advanced (tab)
Click Settings in the Startup and Recovery section, and uncheck the box marked "Automatically restart".
Instead of getting a reboot, you'll get a BSOD giving you information about the problem.
Any money it's drivers of some kind, possibly your modem if you're on broadband. Try connecting it up through a 10/100 Ethernet (NIC).
I don't know how your hardware's set up, but that's certainly a recipe for this kind of problem.
If it's a faulty PSU, though, you may not see the BSOD. Try it and see.
Master of common sense. If you don't like it, stop reading.