Personal Antivirus - got past AVG Free edition and Windows Firewall

on a Dell Pc running AVG Free 8.5 regularly updated and using Windows Firewall (not the best but I didn't think was too bad) but have managed to get infested with 'Personal Antivirus'

Is my protection not adequate? Do I need to buy the full AVG product to get protected? Is the firewall to blame? Should I be running something else alongside or something else entirely?

I going through the 'Read and Run me first' sticky by the way, just wanted a heads up on what went wrong....

cheers
Delarge