AVIs won't play, Windows Explorer crashing (XP) - missing DLL?

[Toronto]

I believe I'm missing a .dll file, because I can't play AVI file via
Windows Media Player (but can via the Divx player, and others.)

As well, Windows Explorer crashes after a while when it's browsing a
directory with .avi files within it - I suspect it's trying to show me
a preview.

Does anyone know which DLL this might be? The error messages I'm
seeing are less than helpful.

(I suspect the dll may have been deleted by an improper software
removal tool that came with a third party video player, by the way.)