I work with viruses on almost a daily basis. I use MSE on Win 7. Pc Cillin on 3 other PC's and Avast free version on another.
I had a pc with Avast on that was brought in today.
Avast detected some viruses but could not clean fake AV 09 and some other. The pc was so bogged down that I could hardly work with it. The customer ignorance regarding removing viruses definitely played a roll. I removed the hard drive and scanned it with MSE. Several baddies were found and cleaned.
I then scanned the same drive with paid for PC Cillin Internet security version.
2 instances of Autorun.ini were found and deleted.
Several issues still had to be sorted: blank device manager page, blank networking page, network adapter not being detected, unable to change option to show hidden files or folders, registry can't be accessed, error messages pop up, can't access msconfig, services.msc services disabled, etc
Ran Avast again on the pc and no further threats were detected.
The point is that just one AV scanner is not enough to detect all threats, free or paid for, not for a casual or reckless users anyhow.:erm: