Btw, in case you haven't tried it already, there's a fix posted on this site at: http://www.daniweb.com/techtalkforums/thread16199.html

You might want to give it a try.

My guess is that M$ have either mistakenly, or correctly, identified your product key or activation code as invalid. They can blacklist a key at any time.
I'm not saying that this applies to you :o...