For somebody who might experiencing Windows Vista (might applicable for other Windows OS variant) takes forever to shutdown, perhaps following tips could help.

• Stubborn process could prevent your Windows Vista to shutdown immediately. What is stubborn process? Stubborn process is application which “refusing” to exit gracefully and Vista wait for sometimes before force to kill it. This process could malware or any legitimate applications. If this “stubborn process” is malware, ensure your endpoint protection up-to-date and recognize the present of this malware in your machine. Combination usage of  tools “Process Explorer” and “HijackThis” might your best friend to identify and reveal the existence of this malware process. If this caused by process of legitimate application, ensure your have the latest update of the application or contact the support if necessary.
• Reducing “WaitToKillServiceTimeout” value to 2 second (2000) might also help. By default Windows Vista will wait for 20 second before decide to kill the stubborn process (WaitToKillServiceTimedout = 20000). Bring up your Registry Editor (Regedit.exe) open and navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control, double click key WaitToKillServiceTimedout and change the value, range 200 (min) to 20000 (max).
• Disabling option “Clear Virtual Memory Paging File at Shutdown” also will help to improve your shutdown. When your pagefile is huge, Vista will takes sometimes to erase it and during this erasing process your disk I/O activity will increase.

Hope that help!