Just tell them that you'll look into the matter and find who is responsable.... This should atleast buy you some time for better trojan scans and FW updates. I've seen peaple share files off of office PCs before so you may want to take a good look at some of your co-workers' activities. If all else fails keep eveything calm by being nice....

In other words send this attachment along with a letter & ask how did they come to the conclusion that you were runing a haven for piracy?

Hummm .... What is your network like? Got any bridged networks that could be running FTP?