(... and no, I am not going to edit the registry. Not this time. I pretend to be an ordinary customer. )
However you want to handle it, or not, is fine.
One thing I can tell you is that you DO NOT want to turn off Vista's UAC if you do get last.fm working. I made that mistake this morning, and when I launched MC12 for the first time, it told me that the last.fm plug-in could not be found or initialized. It required a re-installation of the core MC12 program to get it running again, once I'd restarted the UAC.
I'm going to turn the UAC off again and then reinstall MC12 to see what happens.
EDIT: Ok, so when you turn off UAC (which I really do recommend, if you don't want to have to confirm every move you make twice), you have to reinstall MC12 to restore the functionality of last.fm. But after the reinstallation, you have to also to the registry edit to get it be "Always Active" once again, because it defaults back to "Active when Visible."