Are you downloading movies that have the .avi extention? If so you can convert them to play with Quicktime. Go to http://www.divx.com/software
You should find what you need to convert .avi .mpg etc... to a Quicktime compatable format.
Some movies posted as DivX will play in Quicktime but no audio is available. There's a program called DivX validator that will correct this. Look for it on the DivX site. In many cases that is all that is needed. Good luck.