For Windows Media Player version 11
Open Windows Media Player
Move your mouse over the "Now Playing Tab"
You will see a little arrow, click the arrow and move down and click "More Options"
Click on the "File Types" tab
Check to see if "Windows Media Video file (wmv)" has a tick in the box and hit apply.
Hope that helped.
Edit: In case that doesn't work, what version of FLV player are you using so I can download and check what options it has?
Cheers