Detect where traffic is coming from on my network?

Hi all,
     My ISP is complaining that malicious (viral) traffic is coming from our IP address against other ISP clients, on port 139 and 445. I have since obtained a much better router, through which I can lock down this traffic from leaving the network. However, that being a great solution for the Internet, it is a band-aid solution as far as the source of the infection is concerned. I probably won't have a problem dealing with the malware, but I need to find it, and it's a large network - well, it's only 30 computers spread over a large area, but it's still a needle in a haystack. My ISP has sophisticated equipment to detect this stuff, but does anyone know of any reasonably priced or free tools with which I can scan and detect such traffic or something serving this stuff on my LAN, without having to go to each and every PC? This is something I should probably deal with quickly, so it is fairly urgent - thanks.