There is a problem with clicking on the pop-ups on Vista in that version. Basically, it needs admin access to work, but Avast! wasn't asking for it (via UAC). It's fixed in the latest version (4.7.1043). To update to that, right click the Avast! icon, select Updating, then Program Update. After that, I think you should be able to update to the next version by clicking on the message.
At least, the blue "Virus database has been updated, click for more info" pop-up never showed any more info on clicking because of this problem, and that is now fixed. I assume the program update would have been fixed as well.