Restarting instead of shutting-down

Often, but not always, when I use windows to shut-down my computer it will go through the typical screens of windows shutting down. However, right as it gets to the last step where it would usually "click" and shut-down, it restarts instead, as if I had merely pressed restart.

Information that may be useful:
Windows XP SP2
Asus A7N8X-Deluxe w/ AMD Barton 2500+
420 Watt PSU

Ask for any other information and you shall receive. This computer has worked correctly in this regard for several years, only a few months ago did this nuisance start... and no recent hardware changes (except a switch from a wireless internet card to the on-board wired port).

 

Here go here and see if this helps you solve the problem.

http://www.aumha.org/win5/a/shtdwnxp.php