iMovie doesn't launch

I was working on an iMovie project on my laptop when I decided to copy the iMovie application to my external MyBook drive to have as an emergency backup. When I closed iMovie and then tried to re-open it, I couldn't. I tried launching by clicking the dock icon, the App folder, and even a pre-existing iMovie file, but I keep getting the same pop-up message that reads, "You cannot open the application iMovie because it may be damaged or incomplete." To fix it I have tried everything, including trashing the preference (com.apple.iMovie.plist), updating my software, rebooting, installing the original Tiger Install DVD, repairing permissions and repairing hard drive using Disk Utility. I also tried re-installing my iLife 08 DVD, but of course the iMovie that comes with that package is not compatible with my laptop since it's a G4. I don't know what else to do. Please help. I need to use iMovie. Thank you in advance.