This could be caused by a number of different things. First thing I'd do is open Console.app in Utilities, select All Messages, and search for "quicklook". If that search shows log entries that indicate corrupted Quick Look Generators (.qlgenerator), go find the offending files. Do NOT move them if they're under System. Otherwise, move them to your Desktop, log out and back in, and try again. If you found troublesome Quick Look Generators in the System folder, the recommended fix is to apply the latest Mountain Lion (10.8.5) combo update to your computer.