Windows XP-SP3 won’t shut down

My Windows XP-SP3 goes through the steps when requested to shut down and displays the final notice that “Windows is Shutting Down”, but the PC does not power off. After the normal short period of hard drive activity, it seems to freeze at the final notice and no further hard drive activity is seen. Since this is a desktop PC, I have never turned on sleep mode. I have never experienced anything like this before with any Windows OS. Any suggestions would be appreciated.