I've had this problem before and managed to solve it, but what I think I did then doesn't seem to work now.
When I try to play an AVI file with Nero Showtime, it simply exits without playing the file. All my other players play the AVIs fine (I've tried several AVI files, and several players -- VLC, MPC, WMPv11, and of course Showtime).
AFAIR, last time the problem was a missing codec, and the answer then was to install K-Lite codec pack (which fixed it immediately). That hasn't solved the problem this time, though.
I'm running XP Pro SP2, and Nero Showtime 184.108.40.206. I've just had to replace my hard drive and re-install Windows (which I've done several times before, so I know my way around it reasonably well). This problem MAY have something to do with WMPv11 -- I'm almost sure that NST was playing OK on the current install before I upgraded WMPv9 to v11, but I couldn't swear to it.
What am I doing wrong?