Link to VHD of ISA Server 2006 from Microsoft Downloads.
ISA Server 2006 provides security for corporate applications accessed over the Internet by pre-authenticating users before they gain access to published servers, inspecting even encrypted traffic at the application layer in a stateful manner, and providing automated publishing tools. In addition, by providing HTTP compression, caching of content including software updates, and site-to-site VPN capabilities integrated with application-layer filtering, ISA Server 2006 enables you to securely expand corporate networks. ISA Server 2006 also enables you to manage and protect your network with a hybrid proxy-firewall architecture, deep content inspection, granular policies, and comprehensive alerting and monitoring capabilities.