Just for future reference, it's best *not* to install as admin, as it won't fix anything and can actually cause various problems later on.
It sounds like we really need to figure out what's going on here. Unfortunately I'm still fairly clueless as to why it's happening to a small handful of users, but if we can figure out commonalities between you guys, we might be able to identify the issue. What version of Windows are you guys running?
@jjb_22 What happens if you install to an entirely new folder (temporarily)?