use a windows user with limited right to start setup.
also trying to install to a non-local, mapped drive on a nas.
after start setup and click install, I use a local user to grant admin rights
if I try to "run as admin" I do not have my mapped drive.
I now copied the iTunes folder from network to local, and de-&installed - that worked.
But no the solution, I would expect from "the cloud" apple.
As this worked in the past, it is clearly an new problem by itunes 11.
Win 7 Ultimate x64