Gimme some more info mate? is it working just with some clips and not others? Or is it just not opening any clips?
If it is just not opening any clips at all, and you can open the same clips with another player ie: VLC or Windows Media Player, you probably won't need the codec pack Si mentioned, I would uninstall it and reinstall and see what happens, that worked for me a while back when I had a problem.
If however none of the clips work in any other of your media players it looks like a codec prob after all.