If you have Windows XP and it shuts down slow, and you have NVidia graphic card, this can be problem. When computer starts, automatically starts "NVidia driver helper service", and it don't shut down properly.
1. Go to Control panel- Administrative tools- Services.
2. Right click on NVidia driver helper service, and chose "STOP".
3. On Startup type, chose - "MANUAL".
4. Apply- OK.