Two possibilities:
(1) May be there are some softwares in your PC which takes long time to close
(2) Virus problem
Solution: Use Antivirus which is Update, try to remove the unwanted Softwares, Games,et. When you are shutting down your PC, you manually close all the applications from the system tray(located on the right down corner on the taskbar, just Right Click > Close/Exit....). But first Scan your PC.
