The recently released version 7.5 of...
...claims to have full compatibility with the latest Windows Vista version. So, I upgraded from veresion 7.1 to 7.5 under Windows XP Media Center Edition before starting the Vista upgrade.
After being amazed by seeing Vista Aero Glass features with only shared video RAM, I ran into an AVG pop-up asking me for an activation key. I had never seen that in 7.1 so I wasn't sure what to do next. I finally had to manually terminate the AVG process. Then, I downloaded a fresh copy of AVG Free Edition 7.5 and reinstalled it under Vista. This seemed to fix the problem. No more requests for an activation key.
However, when I booted the Vista PC the next day, AVG crashed at startup. I manually started it and tried test scan. It seemed to work. But, I need to test a few more boot cycles before deciding whether to keep it on the system or replace it with ClamWin (an Open Source anti-virus product).