MP11 crashing when playing AVI files isn't necesarily the fault of the codec.
I've had cases where DivX movies play fine, then without warning, I get a crash. I also thought that this might be a codec problem, but after testing various codec packs on numerous machines, and even doing a complete reload of the os, it was eventually discovered to be the file being played that is killing the codec.
changing the FourCC code from DivX to XVid or vice versa helped.
I haven't had this problem since moving to 64-Bit codecs and using WMP11 64-Bit.
The Vista Codec Package
works miracles. It's the only pack I now have installed.