Google a search for codecs and download a codec pack. There are several and they are free of charge.
Next use another damn player. Try the latest Winamp. Winamp is a hell of a lot better with many more options.
Don't forget that Realmedia (.rm), like Quicktime (.mov) does not play in all players.
Of course .rm can be played in the Helix player outside of the standard Real Player.
There is also a thing called Real Alternative and QT Alternative that are supposed to use the codecs to play the files in other players like Media Player Classic.
Go to
www.videohelp.com to get AVIcodec and/or Gspot to check which codec is required for a file.
Get Codecs Here:
http://www.free-codecs.com
http://www.codecsdownload.com
http://www.click-now.net
http://www.takefiles.com