Your problem may actually be 100% software and not hardware. It could be unstable USB print drivers in combination with XP. In XP, they've pretty much done away with everybody's favorite thing, the "Blue Screen of Death". Instead of the usual "Contact your System Administrator" message, the system just reboots (or turns off) without notice. I would guess that this might be your problem -- the drivers cause instability, and the system just goes down. In that case, try the printer in parallel mode, if it has one.
My other guess would be an electrical short involving your USB bus. I've noticed that some systems have similar problems with any USB device; the system just goes down. Does the system exhibit similar problems with other USB devices? Again, try the printer in parallel mode and see if it clears up, and you might test the system with other USB devices to see if the problem might actually be hardware.
All the best,
-AT