Personally, I don't bother installing protection for the sake of protecting other people's computers. Not because I don't care, but because their machines are so vulnerable that they have to have protection installed anyway.
They are far more likely to get infected by something other than me forwarding them a file with a virus in it. I wouldn't have to know it was a virus or trojan, but I would have to intentionally Email it (whatever "it" is) to them.
But, then again, I'm not involved in the "forward funny attachments to everyone in your address book" game. If I was, I might feel differently.
As far as protecting your own windows partition/vm goes, same answer. Install protection there. It's a better use of your time. If you spend time on the Internet on your windows partition, you should have protection there anyway.