Also, you want to make sure to have WMP11 installed on the machine, which will give you an updated version of the codec.
Can you tell us a little more about what you're trying to do, and perhaps post a link to your current clip? We can offer more specific examples with that.
My compression blog:
http://on10.net/blogs/benwagg/
My compression class at Stanford:
http://digitalmediaacademy.org/courses/video-compression-training.html