Are you running 10.6.3 or higher? Would assume so but never know if you went back to Leopard in between then and now. Just want to make sure.
Also I would suggest deleting the files below and reinstalling.
/Applications/iPhoto.app
/Library/Application Support/iPhoto
Then reinstall from the disc and when you get to the Installation Type menu click on Customize. Make sure you're only installing the help files and iPhoto for now. If you want the rest to be updated to iLife '11 (if you weren't already on it) then you can do that after we get iPhoto working.
Install the iPhoto 9.1 update and then upgrade the iPhoto Library, if necessary, and then run Software Updates.
9.1 Update: http://support.apple.com/kb/dl1322
If you also want the original iPhoto delete the same files from above and put the Install DVD 1 in. Choose "Install Bundled Software Only" and you should be able to install iPhoto at the Installation Type -> Customize window again.