I hardly ever use Windows (thankfully!) but my impression was that it's so vulnerable that you need complete protection including personal firewall. Surely it would make more sense for the Mac + PC version to include antivirus for Mac and Internet Security for Windows? Presumably the best way around this for now (and my plan when my Internet Security trial ends) is to buy Internet Security for Windows and then run the Avast! free Antivirus for Mac beta while waiting for release.
I''ve just installed the trial version of Avast! for Mac on one Lion computer, and the Avast! free Antivirus for Mac beta on another Lion computer (to compare and see whether either runs reliably).