Can't shut down the PC (WinXP Home)

I am running WinXP Home. Recently (about 4 weeks now) my PC keeps booting back up after shut down.

This is what has been happening: To shut down I do the following -
Start - Turn Off Computer - Turn Off.
As soon as it shuts down it boots right back up again instantly.

To shut down without it rebooting, I have to press and hold the power button (on the front of the case) until it turns off the PC. I do this while it is in the shutting down process

I don't know if it is a hardware or software problem. Any ideas? Could it be a virus causing this problem?

Any ideas on what I can do to fix this problem would be GREATLY appreciated?

 

Yes it is.

I will perform the steps you suggested then post again after.