My Packard Bell running Windows XP reboots when I attempt to shutdown. I have re-installed clean software on a clean drive, but the problem remains. The only way of stopping the re-boot is to pull out the plug shortly before re-boot. It must be a BIOS or hardware problem I think. The PC has a MicroStar Inc. MS-6511 1.0 Motherboard and an American Megatrends Inc. 07.00T 0425 BIOS loaded.