Go into your Movies folder in the Finder, and look for the "iMovie Events" folder: inside that folder there is a is an "Event" folder associated with each of your iMovie 7 projects. In each Event folder there is a "Thumbnails" folder containing the generated thumbnails fot that event (project).
Try deleting the offending (crashing) thumbnails or the whole "Thumbnails" folder for the crashing project. It worked for me.
PS: Apple should add a -- cancel -- button on the preview creation sheet, as there is actually no visible/invisible way to cancel or stop preview/thumbnails creation. The preview creation for the added videos was estimated at -- 612 minutes -- on my MacBook core2duo. That is exactly how I realized there was no cancel button, and no working shortcuts either (no escape key, no command-period would work).
Are you seeing an "iMovie Projects" folder in your (user) "Movies" folder? the "iMovie Events" folder is located just above it (in column view).
If by any chance you find the "iMovie Events" folder, look for an "iMovie Thumbnails" anywhere within.
In any case, don't get discouraged, use Spotlight instead. Eheh.
Although I am running a French system, those folders are NOT localized, so you should feed the quoted text to Spotlight and hope for the best., on either a US or UK system.
