fixed my system that would no longer shutdown. After many hours of checking, verifying, and testing I found the issue to be with my Western Digital 1 TB My Book World (WD10000H1NC-00) and Windows 7 Home Premium. The external hard drive is networked and not directly connected to my machine. Windows is up to date as of February 12, 2011 and the WD has the latest firmware (01.02.06 with MioNet built on Thu Oct 21 10:19:41 CST 2010); I was actually running older firmware (01.01.16 with MioNet 126.96.36.199 built on Fri Nov 6 13:28:12 CST 2009) when I had the issue first. I then found the new version when writing this and tested it for this issue and it still happened.
You must access the hard drive in a specific way to not have the issue of not being able to turn off your computer. The below screenshot shows the network window in Windows 7. To access the My Book World hard drive you must click "MYBOOKWORLD" under the Computer section. If you click on it under the Media Devices or Storage sections it will install the drivers (or, attempt to anyway). Once that occurs you must do a System Restore to before that point in time to get your PC to shutdown properly again; you may need to do it through Safe Mode. Good luck!