Can't Play Video - System File Deleted by Mistake

I was deleting some files to get some drivers off my system and I must have deleted something important by accident because now I can't play most videos! I think it was from the Library/Preferences folder. I get an error that says "The movie is not in a format Quicktime can understand." I also can't play any youtube videos.. the video window stays black.. it loads a couple seconds of choppy audio and then stops loading. What can I do to fix this?

Thanks,

-Adam