Sorry--many mpegs cannot be easily played in Quicktime 6.1. The easiest way to see if they'll play at all is to try using the free "vlc" player. You can download it from http://www.videolan.org/vlc/.

If that doesn't work, try qazzaq's idea of searching these forums for many more suggestions. If the file ends in .mpeg, it probably won't play, but .mpg's can usually be viewed.