Desktop constantly reboots

A couple of months ago I installed XP on my son's desktop. Everything was working fine, but now when we turn on the computer, it constantly reboots. I've tryed starting up in safe mode, last known good configuration, all the options available. It runs thru the memory test like usual, then there is a flash of blue screen and it starts all over again. It seems to me he picked up a virus somewhere, but since I can't even log on to run a scan I don't know what to do. It was suggested that I reinstall XP. I put the disc in the drive & the lights flash, so I know the drive is working, but it's not reading the XP installation disc.

Any suggestions? I'm not totally computer illiterate, but I'm not a whiz either.

 School has started and I really want to get his desktop up and running again. Buying a new computer is not an option. I know the desktop can handle XP (it was purchased with a bootlegged version,and running fine with it, but I installed a legit version).

Any help would be greatly appreciated.  :smileyhappy: