PC will not shutdown

I have just loaded XP Professional with SP2 onto my PC and now when i shutdow the PC, windows will shutdown and my monitor shutsdown but my PC is still running and i must physically turn it off via the power button. I am contemplating upgrading my BIOS but as i have not done this in the 3 years that i have had this pc i do not want to kill my pc – any suggestions please. Thankyou