If you try and open quicktime 7 does it crash? If it does I just removed all my codecs from the library and then manually put them back in while reopening quicktime until I found the codec that was crashing it. Then I just deleted it.
I had the same problem and it was a codec issue. I took all my codecs out of my quicktime library folder and then manually put them back until I found the bad one. Takes some time but it solved the problem.