Its probably because a program is not responding to the PC's request to shut down. If you hit ctrl alt delete just before you shut down your PC you'll see a list of programs which are running. (This does assume you're running some version of Windows). You can "end task" each program in turn until you find out which one isn't responding.
There will be a new piece of hardware that has been installed that maybe conflicting with windows quick shut down, which isn't really quick at all.
You can fix this by clicking on run on the start menu, and type MSCONFIG and ckick OK. when the config utility comes on, go to ADVANCED and put a tick beside DISABLE QUICK SHUTDOWN. this will sort it out.