What codecs are these avis using? In QuickTime Player, you should be able to see by hitting command-i or command-j.
AFAIK, anything that worked in QT6 ought to work in QT7 (although 3ivx's performance is much worse, so I recommend installing Divx alone instead of both now). Are you sure all your old codecs are still installed? They should be in "/Library/QuickTime/".