WMV file won't play in Media Player 8 (and before)

Hi,

From an AVI source file, I have created in Movie Maker 2
numerous WMV files that play just fine on my XP box with
Media Player 9. The problem is that these WMV files will
not play on my colleagues Win2000 box using Media Player
7.1 (and maybe on versions up and/or down). I suspect that
the original AVI may have been created with a codec (Real
Player? QuickTime?) that Player 9 can read when the thing
is converted to a WMV, but that earlier versions of Media
Player can not read. Any tips? Thanks! Bob