There's a possibility of conflicting registry entries.
Something like ccleaner may help for that.
First uninstall CCCP,
then download and install ccleaner.
Run ccleaner, click Registry > Scan > Fix
(You can also enhance security on your computer by running the Cleaner from time to time on Windows/Applications. This can clear your internet history, cookies, and passwords-- just makes sure you know what your passwords are otherwise you'll lose them. )
Once you do that, reinstall CCCP, and hopefully videos should play. Otherwise, VLC works most of the time.
Regarding passing .avs scripts onto MPC for playback, I've found that newer versions of MPC may not play them unless in RGB colorspace. This can be fixed by adding converttorgb32() or converttorgb24() to the end of your script and saving.