audio decoder?

when i try to play a video file in winamp it say something about i do not have an audio decoder b/c there was only a 30 day evaluation period. i removed winamp and reinstalled the newest version and still the same thing. now the files open up in the latest version of windows media player and it makes my comp slow and the videos pause a lot. can anyone give me some advice on how to fix this craziness?
~HOP

Do you have plenty memory? This might be the cause of your sluggish problem or just uninstall/rteinstall Windows Media Player.

As far as the decoder, i am sure it tells you what decoder you are missing and then do a google search for it.