Thanks for response, my question was a little bit theoretical, I asked here because I got no answere of same question from avast-support.
In fact it is a good k=question as this might have been the case for a lot of people
The real question is: why do people use Windows on a Mac maschine?
IMHO mostly because of running special/individual Windows-only applications, this means for
offline work, not for online like surfing, mail, chat enz.
Beside this, for me the only
online activity on Windows is to have fully acces to a MS exchange-server 2003(!) by using Internet Explorer, 'cause Safari 4 is compatible only with version 2007.
So: who really needs a Windows virus protection on a Macintosh?

I think, the Mac+PC edition is a network version and not a stand alone version for 2 partitions.