PC won't reboot/shut down

[-zero-]

The past few days I haven't been able to reboot my computer. I'm running XP. I'll go to Start-> Turn Off Computer-> Restart/Shut Down and it'll just shut down a couple of programs and do nothing. I've also tried shutting down from the task manager but that does the same thing. The only way I've been able to power down is by holding the power button on the tower.

Help please?