Test QuickTime to determine if a video can be played. You should test a couple of file types. Chances are that QuickTime will report "Couldn't open the file "FILE NAME" because the file was not found"
If this is the case, Apple reports that the cause of this problem is the presence of a non-alphanumeric character in the volume name of your hard drive. For example a "Hard Drive : 1". Remove that non-alphanumeric character from your volume name and the problem should correct.