To play them just fine on your PC, get the CCCP codec pack (integrates nice with Win Media Player).
http://www.cccp-project.net/
As for playback on Win Mobile, i've always used TCPMP or better yet CorePlayer. I have an SD 750 and never had any issues with the avi files. Matter of fact they're run of the mill types, nothing odd or complex about them.
Use CorePlayer, works like a charm.