lol yeah, it's always the hardware
I've encountered weird crap over the years, just from installing hardware and a good format fixes it, but yeah you should look inside the case and see the total wattage of the PSU and the power of the rails.
It does sound somewhat like a PSU issue, and at the startup is right when the CPU gets a blast of usage so that will indeed tend to cause reboots if it's a PSU issue.
You could run prime95 and if it freezes or reboots as soon as he hits test then I would blame the PSU.
Try the one from your comp in his or something.
EDIT: Oh yeah as far as virus's go, download the usual Ad-aware, Spybot if you haven't already.
has a decent free virus scanner if norton just causes restarts