One of the drawbacks to the Canon S1is's movie mode is that the file format is not usable among all movie editing software programs, at least not without first converting the AVI file to another format.
The dpforum has a good discussion on one person's findings of which software works, and all those that don't -- Video editors for the S1 by Frank43.
Suprisingly enough, the software that works comes from Sony -- and not the big brand names, like Pinnacle. Demo versions of the software available from Sony Media Software. (Problem is that you have to register with Sony just to run the demo -- are they worried your movie making skills will eclipse their studios?)