Technically, yes.
Anti-Virus installed on the host system will not protect any VM's
You will need to install anti-virus in each VM. A VM (virtual machine) is in essence, running a separate OS on a separate box, it is a second separate PC within a PC, so they are not the same or really connected other than shareing the hardware through virtualization.
That should muddy things up pretty good.
In otherwords,,, think of XP Mode and a completely Separate entity which has no relation what-so-ever to Win7, other than they are sharing the same hardware.