Quicktime will not launch when I double-click an (avi format) movie in iPhoto, any ideas?

Try "Command-i" and make sure that the "Open with" area says Quicktime.
Also check to see that quicktime will read other avi files and that you are using the right version of quicktime for the version of iPhoto used to create the movie I beleave that Quick time 7 should work through the current iPhoto. If all else fails reinstall quicktime