Demo video extends to 360, but others throw error

Hi everyone,

first off I've got a fresh install of Win7 Home Premium onto which I've installed the new Divx pack.

I have my machine extending video to the 360 via windows media extender, it plays the demo HD divx file without a hitch. However, I'm getting the "Files needed to display video are not installed or are not working correctly..." error whenever I play a selection of my own MKV files, they're usually divx and are of varying audio codecs.

I've tried installing the cccp codec pack , then on top of that tried installing Haali Media Splitter however it hasn't improved the situation.

What are my next steps in trying to sort this out? Are there any other codedc packs I can try?

Thanks in advance,

Chris