I've been having problems with video playback using the CCCP codec pack which contains Media player classic.
The problem has been that when I use the seekbar in MPC the videoplayback has been choppy for a few seconds. This only seemed to appear while playing backwards. Seeing that MPC is the best mediaplayer out there (my opinion) I knew that I had to get it working properly once more since my old XP system.
What solved the problem for me was just changing the output renderer to EVR (Vista/.Net3) instead of EVR Custom Pres.
For all you wondering how to do this, it can be found under:
-> Main menu -> View -> Options -> Playback -> Output
I've also added a screenshot for your conveniance :