Can’t Find Smurf Attack

Have a small business running Server 2003 SBS as a file server. Somewhere an internal smurf attack has been introduced. Network includes 13 xp-pro machines and the one server. Use mcafee firewall agent I narrowed down the smurf attack to be coming from the server. Downloaded avira-server edition (full functioning) and found no viruses. Also download malwarebytes.org spyware scanner that I use frequently on other sites, but still havent’ found any trace of this attack. Any other way to find out how/where on the server this is happening? Please help