Sounds familiar. Check to see if you have a file called com.apple.xpchelper.cache in your /System/Library/Caches directory. If you're not sure how to get there, go to the Finder, and go to the Go menu, and select Go to Folder... , then type in (or paste) /System/Library/Caches
If it's missing, you'll need to reinstall your OS. Even if the file is there, you may still need to reinstall the OS and/or Aperture if you're having trouble playing or viewing movies.