Neither of you two gave anyone here any information with which to work (hardware, drivers, when it started, etc.), however I do know of one thing that caused this for a high number of nVidia users. In some earlier versions of the WinXP drivers, I think it was 20.xx-22.xx maybe (don't remember exactly), they would load a service called nVidia Driver Helper which would prolong and/or prevent shutdown. You can disable that in services or update to a later driver (23.xx+) and it should fix it if that is what is causing your shutdown issues.