In order to save Replays in TF2, one needs Quicktime. I have several other codecs installed on my system, is there any way to make TF2 use them?
Valve have said in a forum post a while back that they will look into whether they can come up with a different solution for Windows users, but for now we are stuck with it. However, there is an alternative. See this post for more informations:
Use this at your own risk
It is not possible to force TF2 to a completly different codec though.