I have a stable XP OS in my PC. Problem occuring on shutdown. When the OS reaches the final screen of the shutdown sequence, screen freezes, computer locks totally and I have to turn off the power at the mains socket. (neither reset or main switch on PC have any effect.) I am unable to reboot the PC for up to 30minutes and when I turn the mains power on the PC boots from the mains switch rather than the on/off button on the PC. No error messages appear at any stage.