I am emphatically not an IT expert
I am no photographer, so we are getting closer to even (I owe you more I am afraid, since reading your reviews and posts)
Either fusion or parallels can run in a 'shared private mode' (sorry, I forget the exact term the products use) equally well, at least in my experience. Consider this -
At home, you have, in this order:
1. Internet connection (cable, phone line, etc.)
2. Then a router (some call it a modem)
3. Then a cable (or wireless) connecting your router to your computer
It is the router that hides your computer from the Internet. In exactly the same way, when using a shared private network between your Mac and the virtual Windows box, your Mac is a router. So then you have at home:
1. Internet connection (cable, phone line, etc.)
2. Then a router (some call it a modem)
3. Then a cable (or wireless) connecting your router to your Mac
4. Your Mac
5. A virtual cable
6. Your virtual Windows box
The other way to run it is to share the same network that your Mac uses to talk to the Internet, just as if you had a physical Windows box connected to your router (#2 above). Likely not what you want.
When you connect to a public wifi hot spot, that connection is essentially an untrusted network. You can no longer trust the router (#2) above, and so you will want to keep your virtual Windows box behind your Mac (shared private mode).
Sorry about the long winded way to get to your question... but, I think your Windows box, as long as you are behind your home router or Mac, is reasonably safe from anyone trying to connect to you without you connecting to them first. It is only when you are surfing (and sometimes email, although I think that is less so with the excellent AV software out there now) that you may risk falling victim to a virus/trojan. While you can block Windows from the Internet using either virtualization product, I think getting MS updates is a great defense against virus/trojans, so please do not turn off auto-updates. An unpatched Mac is worse than a patched Windows box, security wise.
-Scott