Every movie file has a 4CC code associted with it that tells what codec it should use. You can find out what codec a movie has by using a utility like G-Spot which should also tell you all hte files on your computer that the codec needs, if you copy these files over, you'll also have to run regsvr32 on them to register them. The easier way is to just google the 4CC code from G-Spot and the word "codec" then download the installer for the codec. Alternativly you could just google for "codec pack" and download and install one of them that should contain most of the codecs you need. Be wary of some of these codecs though as many are written to very poor standards and may crash or overwrite some of your other codecs.