Software :: Opening Mov Files On QuickTime?

I digitised some old home videos and all worked very well, except that now some of them won't open on my iMac (OS X 10.5.4). The files are all .mov files and only one of them opens without any problems with Quicktime, with the rest i get a message saying "The movie could not be opened. This is not a movie file". At the Mac support counter I was told that I just need to download a codec and that should solve the problem - but I lost the piece of paper where he wrote down the name of the that codec and I can't remember what it was now... done tons of googling and nothing seems to match.