I''ve had exactly the same problem. I still don't have a long-term solution but I've narrowed the cause down to the release of quick time I'm forced to install if I want iTunes.

Uninstall quick time and poser works perfectly. Unfortunately this means iTunes won't run and I need both. I tried to roll back to an earlier version of iTunes but I guess I don't have one old enough. Right now, my only solution is to keep uninstalling QT and then run the repair utility on iTunes each time I want to use it. I'm sure this is no good for my PC but until I find a better solution it's the best I can do.