What is the point of UAC if not to prevent this kind of thing?
Despite the risk of the Dreaded Down Vote, I decided to use the "Answer" format to post what is basically just some questions better suited for a comment.
As has already been observed in a comment by Qwerty, the UAC isn't much use if it is either (1) disabled or (2) the warning is ignored and a rogue program is given the authority to do whatever. My understanding was that if you had UAC enabled (... did you check it's current setting?) then you could not have been infected unless someone authorized the rogue program.
You mentioned that "home PCs need to be used by wife, kids, friends, babysitters etc.". Could one of them have let whatever this is through the UAC? (Frankly, the thought of "kids" or "babysitters" using a system other than via a limited user level account without any administrative/install authority makes me shudder. BWTHDIK?)
I think another possible way it could happen is if somehow a program was given Windows XP compatibility and that program then turned around and bit you either directly or because it was somehow subverted. Windows XP compatibility seems to give some form of elevated authority to a program. Could that be possible in your case?
Another part of the reason to post this is because if I'm wrong about UAC needing to be consciously by-passed in order for something to infect a Windows 7 system, I'd like to know more about that threat. At the moment I can't see how it would be possible though.