Scanning outgoing internet traffic from public wireless users

Hi

I have public users that come in and use our wireless. Our ISP has contacted us saying they have picked up mulitple viruses coming from us.
Our firewall scans incoming data but not outgoing and is unable to. I am going to setup a computer between the wireless controller and the firewall that will scan all the outgoing internet traffic.

Question is, what software is out there that would work for this situation?
Anybody have a similar setup?

Thanks

So you don't mind people jumping on your wireless connection? Have you considered fixing that?

No no, this is a library that has public internet use for our patrons.

Oooops. Sorry. My Bad.

Even my free AVAST will scan outgoing mail...I always turn it off since I'm certain I'm virus free...But I'm in a wired enviroment.

IMO the best approach is to place a *nix host running software such as Clam to scan all traffic - incoming and outgoing between your network and the internet. Care must be taken to size the setup properly or it can become a bottleneck.