Quicktime 7.6 on Mac OS X 10.5.6 does not play most video files

I got an iMac for Christmas (Mac OS X 10.5.6) and since then I've been trying to get my video files to work in Quicktime, but 90% of them don't work. The only file type that seems to work most of the time are .mp4 files. I download a lot of anime (usually .avi or .mov) but when I try to play them in Quicktime, I either get a black screen with just the audio, or just the image but no audio. I've downloaded all the components Quicktime told me to and I've made sure all the file types are checked in the preferences. Nothing has worked, not even uninstalling and reinstalling Quicktime... Please help!

Message was edited by: VuduPins

Message was edited by: VuduPins