Why Windows takes so long to Shutdown ?
This can happen when third party or Microsoft applications have not properly unload from memory when exiting. Windows will keep trying to completely close them until finally decides that it can't and should shutdown. Even if you find the application causing the problem it may be impossible to do anything about it.
To fix this problem Microsoft released the User Profile Hive Cleanup Service
After the installation open up your services list (Click start, then run and type services.msc now click OK), you should see new service running in the background:
This free utility automatically cleans up user profiles reclaiming resources as soon as a task is finished and prevents you from wait when shutting down Windows XP.
The other option is to download this .zip file.
Running the .cmd file contained will force Windows XP to shoutdown in few seconds.