iMovie is part of iLife and came bundled with your Mac. If you have deleted the application you will need to reinstall it from your original install disks, then update via Software Update.
If you also lost your original install disks:
If you are in the US call Apple Customer Support 1-800-767-2775, provide the Serial Number and specifications of the Mac, and for a reasonable fee, they will supply a replacement set of system discs (if available).
The discs will be for the original version of the OS that was pre-installed when the Mac was manufactured.
If you are in another country:
http://support.apple.com/kb/HE57
Message was edited by: Klaus1