You've made a movie with The Sims 2, you want to show all your friends so you stick it on disc and take it round their house.
They don't have The Sims 2 and your shocked to find you can't play the videos.. why not?
Well it's a simple answer: EA decided to use a codec made by a company called On2.. The codec isn't really all that common and if you don't have The Sims 2 chances are you don't have the codec.
The solution is simply to install the codec on your friends computer and they'll be able to watch your videos. You can get the codec from the following page: http://www.on2.com/downloads/vp6-decoder/