My favorite new software for converting avi and mov files to Quicktime or Mp4s for iMovie is called Visual Hub.
I have paid to download countless Quicktime pro and WMV converters, and most of them frustrate me endlessly. Visual Hub is easy (drag and drop) and hasn’t failed me yet. It also has a free trial without annoying watermarks or frequency limitations (although it does limit duration to under 2 minutes).
Most importantly, when it’s done its work, it boasts a lovely “ding” that sounds like a toaster. I sometimes salivate when I hear that ding.