Linux Security has an article entitled "An Introduction to Nessus" which is a decent read. For those not in the know, Nessus is an open source vulnerability scanner.
Some organizations use it instead of ISS as it's attack database is generally larger and more up-to-date. The drawback is that it also can do damage in it's penetration testing if you're not careful (there are switches to disable the more brutish attacks).
Update: Bowulf has a piece in which he indicates that you can avoid the setup and configuration of Linux and Nessus by using Knoppix STD. The only thing you have to worry about otherwise is gathering the updated NASL signature files.
Hint: you can add them to the distribution prior to buring the iso by mounting it via the loopback device. (If there's enough room.) For Linux, try
mount cdimage.raw -r -t iso9660 -o loop /mnt