cannot shut down the PC unless

My PC suddenly have following problem:

Whenever I shut down my PC ( Win XP), the PC is shut down successfully. Then after a few seconds, it auto start again without touch any key? Also there is no button out there I can perss to shut the PC down. At moment, the only way to shut down the PC is to unplug the power line.

Any Clue to the problem?

Thanks for your help.