No Windows Desktop OS supports multiple Remote Desktop Connections simultaneously. That is to say, that the console session is surrendered when a remote destop session is initiated. For that you would need something running Terminal Services, which "legally" is only support on the Windows Server platforms.
The discussion seems to be about three different levels of computing.
Badrobot's device basically shares computing power and perhaps shares access to files, but each user is essentially working on his own computer.
The next step up, is using the Mac to remotely log on as a user on the Vista PC at the same time another user is physically logged on to the Vista, and both use the Vista concurrently doing different things, independently and unaware of each others activity.
The third level Is something like TeamViewer that Trouble suggested--collaborative or interactive computing, where both users are aware of each others activity and share a common view, and each can do independent things in relation to the shared view.
Interesting that products exist to do 1 and 3 but the middle one, which sounds like buggs1a's objective, can't be done, essentially because of licensing issues?