UPDATE: - In Backtrack 5 R1 you will need to additionally download and install Nessus, which I have briefly described in the following post
The rest of the setup
1) Get a free home-use key on the Tenable/Nessus website
2) Enter the key as follows
/opt/nessus/bin/nessus-fetch --register xxxx-xxxx-xxxx-xxxx
3) Create a user and password (and hit enter to skip the rules)
4) Start the service
5) Start the scan, and view the report
The Nessus user-interface is so straight-forward that don't think there is any point in me describing where to click or what to put in. Just play with it for a minute or two and you should see how it works.
Using Nessus to scan a set of machines really is a no-brainer. Here is a sample report (This XP systems needs patching ;o)
Whilst this is no substitute for a Penetration test, a basic vulnerability scan can certainly help identify computers that are missing patches, or have poor configurations.