First thing to do.
Most Win XP shutdown problems have been that it reboots when shutdown is attempted. XP executes an automatic restart in the event of a system failure. Anything compromising the operating system during the shutdown process could force this reboot.
Disabling the “restart on system failure” feature may permit the exact cause to be isolated: Right-click on My Computer, click Properties, click the Advanced tab. Under “Startup & Recovery,” click Settings. Under “System Failure,” uncheck the box in front of “Automatically restart.”
When you have done this try shutting down the computer. You should get a blue screen error. Write down the error( word for word exactly), then look up the error at the aumha website.http://www.aumha.org/a/stop.php
There you can read up on what may be the problem. If you do not understand what you are reading please post the error here so i may look it up.
Another thing i would like to know is what kind of accelerator they installed? Was it a video card accelerator, or a system one.
Have you read these Microsoft articles?http://support.microsoft.com/kb/308029
Please don't try fixing anything until you get a answer here as some things at these websites can do more damage if it is the wrong fix for your problem.