Forget viruses, worms, and rootkits.  If hackers really wanted to damage businesses and Microsoft it looks like the WGA server is the single point of failure.

http://computerworld.com/action/article.do?command=viewArticleBasic&articleId=9032798

Once a hacker takes down the WGA server all he has to do is wait until the millions of Windows machines phone home.  The (de)activation controls built into Windows do the rest. 

Couple that with the security and reliability problems associated with Windows servers and this could get nasty. 

The moral of the story:  If you must use Windows, make sure you have a fully functional backup machine running a non-Windows OS.

The backup machine could be anything from a cheap old computer running Linux to a new Mac.  It could also be a Linux partition on your hard drive.  (The partition is probably better than having a separate computer.  If Windows goes down and you get locked out completely, Linux will still let you browse and copy the files from your Windows partition.)