Quik Time seriously messes with the efficiency of PC's. It runs in the background, constantly trying to play all your files for you. If you only load the small, viewer version, it frequently wakes up, goes out on the internet and tries to download the latest upgrade: Usually at the worst possible moment. System programs do this, too, but you can control when they go shopping.
I uninstalled all quiktime files from my PC and haven't had a render crash since.
Flash works fine on PC's, only running when needed.
Now I have to load a backup computer with quiktime to watch Mark's super secret dark art tutorials.