Is there a way to sniff out Netsky and Beagle Traffic on a network to find out where its coming from?

Every day I get an email from the same people. The emails have attachments with Netsky or Beagle on them. I have NAV Cor 8. I am worried about the other 30 clients on my office.

I am having trouble hunting this thing down. I have run online scans on all the boxes in the network. Cant seem to nail this thing down.

Any suggestions would be helpful