There was a similar thread on this subject here: http://forum.avast.com/index.php?topic=74135.0
Potentially Unwanted Programs are potentially unwanted as the user of the computer did not install it willfully and knowingly. If that isn’t the case the program can be considered a risktool.
Had the program been installed, then a System restore to a point before the infection occured in Safe Mode also had been a secure way of removing a Win32:Pup-gen infection. A full avast bootscan should then follow this procedure,
polonus