Its always been fairly quick. One of the first things I disd when it started to slow down was to open up and clean, normally do it once or twice a year anyway on all the PC,s in the house.
I have googled "slow shut down" and have a few options I can try when I get on to my home PC ( at work atm)
I will also try a registry clean up, see if that improves things. I will update if it works so others can see the cure (hopefully!!)