I used this one. http://www.dynamicdrive.com/dynamicindex3/snow.htm
I didn't want it on all the forums so I just placed the code in the unique header section under the general forum permissions. Now, the only place it'll appear is under the general forum.
You got to change the location of the .gif image though or it will not work. By doing it this way, it will also not appear once you click on the topic you want to read. This way, it's not a major "distraction" once you are reading the topic.
Added some lights to the top for Christmas as well. Here's a snapshot.