True. In which case I'd recommend almost anything but Norton. Strictly speaking though, a PC without antivirus software will probably get pretty b0rked (sorry, just overrun by l33t speak momentarily, it won't happen again) irrespective of any incoming emails from Macs.
I'd say there's a greater risk from direct file-sharing which, to the best of my knowledge, bypasses the PC antivirus software.
Use ClamXav -- available on Macupdate.com (or do a Google search). O'Reilly just had a great write-up on it and its a FREE open-source program, full OS X compliant and even has a background agent. I've set it up to do live scans of my Desktop, inBox, and my downloads files. This way I won't be passing on "bad" files to my Window's friends --> don't want them blaming my Mac for a virus they got!