A really simple thing to try is starting your PC up in safe mode (press F8 while the machine loads, a split second before the windows 'loading bar' appears) and choose the safe mode option.
Can you shut down correctly in safe mode? If the answer is yes then the problem is with an application or driver on your PC that is being loaded when the PC boots normally.
Most commonly this can be display or sound drivers or security software like antivirus, firewalls or anti-spyware. I'd start with security software. Completely shut down all the security software you have (making sure it's not running in the background, you can kill that stuff from task manager if needed) and you can see if that does the trick.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:220.127.116.11) Gecko/20091102 Firefox/3.5.5 (.NET CLR 3.5.30729)
Core i7 920 | ASUS P6T Deluxe v2 | 3TB+ HDD | 12GB Corsair DDR3 | Radeon 4890 Xfire | X-Fi Titanium Fatal1ty | Logitech Z-5500 Speakers | Dell 3008WFP | Seven RC1