How do I do this?
Example - because when I open a video in WMP and then go to properties, it says which codec they use. How do I change that?

Directshow works on a priority system. If you want a different filter to be used in place of another then you need to make sure that it has the higher priority. You can use a filter manager like Radlight's or a player that supports filter over rides, etc. If you never plan on using a filter then you can just unregister/remove it.

Hi,

I might have misunderstood but what I think you need is two tools called Avi4cc or Div2Mp4. These are the only two utilities I know that can change the codec used to play a certain file. Since all avi files are the same except for the codec used(same container) they are all basically interchangeable except a few which use a different filter altogether(So this is basically trial and error-i.e. note down old codec, change codec and play. If it works, problem solved otherwise change to something else or old codec[original]).

Hope this helps.
Unicorn23'.

P.S. Sorry if this is not what you were looking for.