Real AVG compromised by imposter

Hello,
A week ago, a pop-up came on screen that looked enough like real AVG to fool another computer user here into clicking on it. Turned out to be Personal Antivirus. I've tried all sorts of things to banish it but it seems to be lurking around in my machine. Now AVG 8.5 won't update. I went searching for files called "PAV" and read some of the tips on this blog and searched for Uninstall choices. No PAV strands in Uninstall folder but something called "Rapid Antivirus" turned up -- probably the thing that was giving me trouble a few months ago. I'm uneasy messing around in the registry so am looking for another fix. Any ideas on how to get AVG updates and how to get rid of these fake antivirus programs that have infested my computer??? Many thanks.