Load up regedit (Start -> Run -> regedit) and follow this path HKEY_LOCAL_MACHINE -> SYSTEM -> CurrentControlSet -> Control. From there find the WaitToKillServiceTimeOut string. Change its data from 20000 (default) to something lower like 10000 or even 5000. Please keep in mind that the computer value used here is milliseconds. After that's done close regedit and reboot your computer.
After this change has been made, your computer will shut down much faster!