I am also struggling with this problem. I have only used Avast on this machine and have updated as prompted for new versions by the program.
Running Windows 7 Home Premium, 64-bit version, using Avast Free version 8.0.1497.
I have performed the standard recommendations several times, used aswclear in safe mode etc., no success, still getting the annoying popup every time I start the laptop up.
I cannot find any traces of older Avast versions anywhere on the computer. What now ?