Maybe because he wants to send those files directly from linux, not from windows. I can say it's a pain to restart into windows ;-)
Also, this is not quite the right place to talk abou this :-( This forum's purpose is to help aMule users on Linux.
Anyway, AFAIK, all good antivirus for Linux are commercial. Check Tucows if you're realy interested in this issue. On the other hand... why don't your Win-friends use an antivirus if THEY're the ones who really suffer the damage? :-D
Greetings