I had a similar problem with another program a while back. It does indeed have to do with the installshield program. What fixed it for me was renaming the installshield-folder (c:\program files\common files\installshield). Just put an x or something in front of it and re-install Studio. Hope that does the trick for you.

Have you tried right-clicking the install-file and selecting ''Run as administrator'' ? I'm running Vista Home Premium (32bit) and while I'm logged in as an administrator some programs still require this method of executing a file. I haven't been able to try Studio Plus 12, just ordered it moments ago...but I don't recall having any issue's ...