Someone currently has the W32/Mytob-AR virus on their PC. This virus takes an email address on that person's machine, which in this case happens to be the email for this site, and sends emails containing virus attachments from that address to other people in the address book. It is easy to fake someone else's address unfortunately.
It is not the fault of this site that one of its email correspondents has picked up a virus.
http://securityresponse.symantec.com/avcenter/venc/data/w32.mytob.ar@mm.html