I’m not a regular poster on any boards, but I too suffered from the QuickTime playback issue (running Vista Ultimate on a mobo with VIA chipset, not nVidia.) Fortunately, I spent ONLY a week banging my head, until I found this thread. The breakthrough was testing a .mov playback on a USB or non-SATA CD drive, as others here have suggested, and NOT having the problem.
The solution for me involved installing VIA HyperionPro v513a chipset drivers in a certain order (don’t ask me what order finally worked. I don’t have a clue.) My SATA drives had worked fine with all other apps, but for some reason not with QuickTime, and apparently needed the very latest drivers to fly. All is peachy now. Hope this helps…