Usually a slow shutdown is caused by a program so try:
Check for a bad program:
Go to Services tab and click on the box to hide Microsoft Services then uncheck
everything that remains. Go to Startup tab and uncheck everything. OK and
reboot. THe changes you make will not do anything until after a reset so you only the second shutdown might improve. If it doesn't shutdown faster then go back into msconfig and recheck the
things you turned off. If it helps then go back and turn on a few items each
time until you find the culprit.