windows media player not working

On my MacBook Pro I suddenly can no longer play any videos done in windows media player. Up until a week ago I was fine but now I get a message saying, "This file may not play correctly because it was compressed by using a codec that is not supported". I have downloaded Windows media player anew and instlalled it but no good. I can take a video from my other mac that plays OK on it but when I put it on the MacBookPro I get the message. The video will play if I click OK on the message but there is no sound. Any suggestions???

 

Thanks, I downloaded it and all my videos play. Only thing is I have to open t hem through the Flip4Mac. If I double click the video itself I get an errror message. Normal????

 

It said something about purchasing a serial number or something so I could upgrade. Will I need to do this to get it to work at this level?