There's no use in even trying. My grandparents had the same problem. Easier just to stop by every 6 months and format their hard drive.
Actually something like Norton System Works seems to protect better for the computer illiterate.
I absolutely HATE it and wouldn't let a copy of it even sit beside my own computer, but for people with no clue it seems to stop a lot of stuff. It's definitely more aggressive than AVG. You might want to try something like that.