the free edition.
I'm running Windows 7 Ultimate and that has ZoneAlarm as its anti-virus and firewall, but on this machine I have installed Virtual PC and XP Mode. This allows me to run a version of XP from within Windows 7.
It is fully functional and has its own access to the internet through Win 7, but my research suggested that, even though it was running behind Win 7's firewall and anti-virus, it still need its own firewall and AV installed. I am using the Windows firewall on this XP and I needed a lightweight anti-virus suitable for an OS within an OS.
That's why I chose Panda Cloud AV free, and it seems to be working well.