Yes but Apple trailers in XBMC (other media center software) are working great with K-lite, so maybe You can manage to make it working with Mediaportal too
I'll be reading the thread later meanwhile could You post which of K-lite codecs shouldn't be installed??
Please read all my previous posts on this issue in this thread. I had exactly the same problem. I ended up uninstalling K-lite codec pack and installing Vista Codec pack. This worked perfectly. Please note Vista codec pack works in XP.
Cheers,
Colin