|divbrain, here are things to check.|
Start > Control Panel > Power Options
Advanced Power Management.
Tick > Enable Advanced Power Management support.
Does your machine refuse to shutoff at shutdown?
This may or may not work depending on whether or not your system supports APMS. Start regedit, and make your way to [HKEY_CURRENT_USER\Control Panel\Desktop] and in the right pane look for the key PowerOffActive. Change the value to 1.
Will not Shutdown
When booting press delete and go into bios and then power management, look for the power down or shut off option and select instant off instead of delay etc.
In Windows XP there is a bug that keeps the computer from powering off on shutdown. It is easily fixed.
Click on Start/Settings/Control Panel/System.
Click on Hardware then Device Manager.
Click on View at top then Show Hidden Devices.
Click on the plus sign next to NT/Apm/Legacy Support.
RIGHT click on the entry NT/Apm/Legacy Interface Node.
Click on Enable.
Resources for Troubleshooting Startup Problems in Windows XP