PC reboots after keypress on screensaver

Hello,
I have a PC running XP pro SP2. The screen saver kicks in after two minutes. When i move the mouse or press a key to get rid of the screen saver, the PC reboots. The screen saver is one of the standard windows screen saver and not anything loaded. I have scanned the machine for a virus but it’s clean. Anyone out there ever had this problem?