One software I was/am still working on has an onscreen display(clear always top window ) so I can draw icons and text over a game, it's a pain to alt-tab out of(due to me setting it to constantly check to see if it's ontop, and if not, to set it).
So I would say that it's certainly possible, although I haven't tried specifically to do that to emulate UAC.
Edit:
Infact I had a bug at one stage where if I closed the main window, the invisible window would remain running, with no entry in the start bar.
Now I'm becoming a little more concerned, as I could also listen for hotkeys, (such as Ctrl alt delete) and display my own 'secure login' page. Shit
So I would say that it's certainly possible, although I haven't tried specifically to do that to emulate UAC.
Edit: Infact I had a bug at one stage where if I closed the main window, the invisible window would remain running, with no entry in the start bar.
Now I'm becoming a little more concerned, as I could also listen for hotkeys, (such as Ctrl alt delete) and display my own 'secure login' page. Shit