1. QuickTime 7 is outdated, as QuickTime X is the current version.
2. QuickTime X doesn't have a Pro version.
3. The registered (Pro) version probably won't even allow you to play the AVI files you want.
4. You can get
Perian and play any file you want in QuickTime.
5. If you don't want to use QuickTime, get either
VLC or
Mplayer.