If your launching the files from LW, then disable the LW player. In LW go to Tools>Options>Player, untick the box and click apply.
Now if WMP is set as your default player then your files should launch in WMP. If not then you'll need to make it you default player. See:
Setting Default video app
If they still won't play then it's probably a codec issue. See:
CODECs list for Movies /Alternate players / Video help forums.
Consider trying the
VLC media player, it has the codecs built in and will play most files.