Quicktime Converting?

Hello,

I have Quicktime files from 2004 and they are in .mov format. When I try to play them QT will open up and I get a loading bar and it says "converting". I am not sure what is being converted? It takes about 5 min or more. Other apps running on my computer starts to act as if they are in slow motion.

When its done the file plays perfectly fine. When I try to close it, it says if I want to save the converted file. So I chose yes. I got an 8GB FILE!! The original file is only 117mb and is about 30 min long.

Any one know whats going on here?