App Install Preventing Shutdown

Every time I try to shut down my computer a message pops up saying my computer is in the middle of installing iMovie and it will damage my computer if I interrupt the install. This message prevents shutdown and the "install" has been stuck "installing" for 3 weeks. I have tried Force Quit, cancelling the activity in Activity Monitor, and I've even uninstalled iMovie, but this message keeps popping up (and reinstalling iMovie...). Would anyone have any ideas on how I can clear this "install" so I can shutdown my computer the proper way (without just holding the power button down until it forces itself off)? Thanks so much!