I have this problem also, I tried resetting the security center but it didn't do anything seemingly. I tried using a command prompt way where I tested it and it came back as fine...then I refreshed it and it basically errored, but did refresh it as it refreshed the wee icon saying the av was turned off (the balloon notification happened again).
Do you or any one who has had success have a definitive way of resetting the security center with vista? I see there are multiple ways.
As a side note I even refreshed vista back to an earlier restore point to get back to the previous version of avast and it didn't have any issues at all (aside from the avast program being out of date, but recognized avast as on in security center etc) as soon as I updated to the latest version of the program it produced the security center warning again (despite avast reporting as up to date and active).
It may be possible there is something missing in the update that isn't talking to windows vista and alike? My Windows 10 machines are fine thankfully.