The shutdown window keeps coming up and my...

Question

No matter what I'm doing at the time, my computer will show the window to turn off the computer as if I clicked on the start icon and selected the turn off icon. If I don't click cancel fast enough, the computer shuts down. It also starts itself up too. Some times it will not happen for more than an hour, but most of the time it's almost constant. I am using windowsXP home version. Please help me with this as I am ready to take a hammer to it :-) thank you