Vote for avast! ISA Server Edition
Antivirus solution for your MS ISA Servers (also works with MS Proxy Server 2.0). Provides real-time scanning of HTTP and FTP traffic at the gateway. avast! ISA Server Edition provides protection for Microsoft ISA 2000 Servers (and also works with MS Proxy Server 2.x). Its main features include scanning, scalability and reporting capabilities. It integrates with ISA Servers via Microsoft's own "ISAPI" interface.
The product works as a plug-in for avast! Server Edition. Therefore, it uses the same engine as the Server Editon itself.