I'm now also experiencing this issue, on two machines, and in both cases the last thing I installed successfully was Adobe Reader 9. Could that be it?
I'm sure it isn't Adobe Reader 9, it's something MS did because I installed Quicktime on another windows
7 that I had put on another PC and the install worked flawlessly. I installed dozens of applications before Quicktime (some heavy ones like visual studio, vmware, office, jdk, eclipse, premiere cs4) including Acrobat 9 that was the first I installed and none
of them gave this issue. This issue started appearing 3 days ago as the boom of posts about that issue started the 18th of january proves, I've had the Windows 7 7000 build installed on the other PC since it was leaked almost 3 weeks ago and I never had this
issue there, most likely because that PC wasn't connected to the internet during that time.