This is a well-known problem with QuickTime & Vista (all answered in our Download Video FAQ
When all else fails, use VLC Media Player
Also see this particular FAQ:
Q: When I try to play a video the sound is absolutely fine, but the screen just shows green/black fuzziness
A: Open QuickTime 7
Click on Edit tab and go to Preference,
Then click on QuickTime Preference.
Once that opens, click on Advanced tab
You will see a selection for video or Direct X : unclick the Direct X radio button
Click on Safe mode (GDI only)
Then click on Apply, then OK.
Restart QuickTime and play a movie to test.