Well you can re-download it then. Did you do a safe boot & repair permissions? ie: reboot whilst holding down the SHIFT key until you see the SAFE boot option. After started up, go to Utilities folder, open DIsk Utility & repair permissions. Then reboot normally.

If that doesn't fix it, then try the last step in the list here http://www.gnutellaforums.com/showth...threadid=36374

If Quicktime is trying to open the file, then you have a system problem & you will probably have this problem with other program installers. You may need to repair disk using Disk Utility from the OSX installation disk.