Have you tried trashing the iPhoto preferences file?
~/Library/Preferences/com.apple.iPhoto.plist
You can move it to your Desktop if you want to keep it, but, if it's the problem, you don't want to restore it anyway. Don't trash it until you move it to the Desktop, launch iPhoto, and see if everything works. If it does not, it's probably not the file, and you can move it back.
Also, try repairing permissions via Disk Utility.