As of yet, there is not official Microsoft patch for the vulnerability and most anti-virus software is not capable of protecting against it. Microsoft has recommended a work around; however, several security experts do not believe the work around is enough to protect against the problem. A tool from a third party has been created to deal with this problem and it has been reviewed by the security industry as an acceptable fix until Microsoft issues an official patch.
Unlike most past security problems with PCs, this particular one can not be avoided. It is possible for your computer to become infected under circumstances that would normally be considered "safe". If you E-mail client or Instant Messenger program saves attachments sent to you to your hard drive (as most of those clients do), and if an "infected" attachment is sent to you, when that file is saved to your hard drive you are then at risk. Any of the common tools and applications that index your PC's hard drives for faster file searching/sorting can trigger the payload in the file. The other common means for this infection to occur is if you visit a website that contains one of these media files, seen to be often masquerading as an image. When your browser attempts to view the file, it then triggers the infection. It's that easy. Past methods for avoiding infections - cautious browsing, having an updated AV client, firewalls - none of those are capable of preventing this problem at present without serious degradation of your online capabilities.
Please read the official statement from Microsoft and a more layman's analysis of the problem, including a recommended "fix" (second link), from the links below. As with any security notice, check several sources for corroborating information. Pass this on to friends and family.
Happy and *Safe* computing in the New Year.

Microsoft Security Advisory (912840)
http://www.microsoft.com/technet/securi ... 12840.mspx
Windows WMF Vulnerability News & Updates
http://www.grc.com/sn/notes-020.htm
Computerworld Articles regarding this problem:
http://tinyurl.com/caqwj
http://tinyurl.com/bvvqw
http://tinyurl.com/ch7zd