Make a note of the time of shutdown when this next happens then when you reboot, check the Event Viewer for Windows logs for both Application and System. Check the times to see if anything is causing such a long shut down period.
Running a Dell Studio XPS 9100 with Win 7 64 bit. Up until recently it would shut down within 10 seconds. Lately it has been taking 5 minutes or more to shut down. This is not like the other problem I have seen in the forum, where there is a shut down and then it restarts itself. Have checked for viruses, defragged it, removed recently installed programs, etc. Still have the problem. Don't see anything unusual running before trying to shut down. Can't tell what the system is trying to close once the shut down process is started. Help would be appreciated.