Hmm, I ran into something similar and I found that it is caused by the "customer experince improvments" reports (aka SQM). Try turning it off and see if it works (if you haven't reinstalled already)
How to turn it off:
1) search the control panel or start menu for "customer experience", you should find a task called "Change Customer Experience Improvement Program settings"
2) open that task and select "No, I don't want.... "
Probably you should turn that back one after you're done installing, what's the point of a beta if the errors and other problems are not reported?
Thanks, it was the customer experience program just like you said: Windows update and all windows installer based setups started working again after turning that off. I hope they'll release an update for the issue soon.
"Why would anyone install QuickTime?"
To see the exclusive movie trailers that are available at decent quality only on the Apple website or to use iTunes that is required to sync the most popular mp3 player (ipod) and the most popular phone (iphone). I can't believe a beta could break so easily
with an application so popular: the Quicktime setup didn't even launch yet win7 was instantly broken.