If you're running Virtual Server (or Virtual PC), and have some guest machines connected to the Internet, you probably don't want to leave them unprotected. You may think that installing ISA on the host machine would protect the guest machines. But it doesn't! You can verify it easily - run some traffic between the guest machine and the Internet (say, browse to some public web site), and see that the traffic passes even though there's no rule that would allow it. Also, the traffic does not appear in the ISA log at all.
The reason for this is that Virtual Server uses an NDIS driver to route traffic to its guest machines, according to their MAC addresses. Since NDIS drivers are located below ISA's driver (fweng.sys), the traffic is routed before ISA even sees it:
One way you can accomplish this idea is to have another NIC (call it Internal), connect the guest machines only to that NIC, and have ISA route/NAT traffic between that NIC and the "real" (External) NIC:
Actually, in this case the guest machines are no different than other physical machines connected to the Internal NIC. You get all the hassles of having another network - IP address assignment, NAT, etc. - but at least your guest machines are protected, and you've only used one physical machine! For extra virtualization credit, you can use a loopback adapter for the Internal NIC.
ISA Server Sustained Engineering Team