MPEG2 decoder

Hi There - I have been downloading mov files from my video camera and they having been playing fine in quicktime. Today i bought Final Cut express and downloaded some new files from my camera only to find that they were downloading as MPEGs and would play in Final cut or Quicktime. So I purchased the mpeg decoder from the mac website. Now I can see the MPEG segments but they are very grainy and no sound. All my previous mov files no play very grainy. So I want to know how I can uninstall this decoder .. I tried uninstalling quicktime but the new decoder persists and the new segments load in itunes... Can someone .. anyone tell me how to install this decoder ....
and if anyone knows how can I export my segments in mov format again I'm not sure how this changed .. whether it is a setting on my camera or on my mac.

Please help