Windows Vista Can't re-activate Windows Vista!

Hi. I turned on my Vista Ultimate computer today and clicked my user account. Before my destop loaded I got this lovely message saying that I need to activate windows. What the heck? Activate windows? Are you serious? My computer has been activated since I bought Vista 1 and a half years ago. How did Vista just decide that my computer wasn't activated? When I click Activate Windows, it fails and I get error code OXCOO4EOO3. When I click more information about the error code it says The Software Licensing Service reported that license evaluation failed. I ended up recovering from reduced functionality mode by starting up Windows explorer but I can't re-activate windows. Could I have been hit by a virus that caused this problem to happen? The only thing I changed on my computer two days ago when it was working fine was that I installed a game called Microsoft Tinker from Windows Update. Should I call Microsoft and tell them what happened or is there another solution to get my system re-activated? Out of all the weird and puzzling errors that I have seen on a Windows system I have never seen a computer just un activate itself.

 

Thanks for the link Mitchell. Read through it and was able to perform a system restore two 2 days ago. I just finished re-updating my security software since I had to do it again. Windows shows its been activated so I hope I won't have this problem again. Its still very puzzling why this problem happened in the first place. Anyway my system is back up and all is well.