Windows XP computer will not power down.

"Control Panel"
then "Power Options"

There should be some settings you can play around with until buttons do what they're supposed to.

That's the first thing I'd check :)
Happy hunting.

http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/russel_02may13.mspx

One of the first things you'll find searching, is the suggestion to make sure that the option to automatically restart on system failure (Blue Screen Of Death) is *not* selected. Its not so much that this is the cause of your problems, but rather the outcome of trying to shutdown, and having a system crash when a driver is unloading.

The link above points to the other options... but try deselecting automatically restart if its still selected.. deselect it, reboot the pc, make sure it starts up and shows the "automatically restart" as unselected *before* you attempt a straight "shut down".