If you're open on the Snow Leopard side and you have Windows open and minimized, wouldn't any virus attack see the Windows file?Your Windows VM can't get infected by what you're doing on the Mac host computer, and vise versa. (Actually, it is theoretically possible that someone could write a Mac virus that hunted down the Windows VM and infect it, but AFAIK no one has ever developed such software).
They really act like completely separate computers. The only significant difference is that the Mac host computer may (depending on how the network is configured) act as an additional firewall between the Windows VM and the Internet.