Online Armor Free
Like Comodo Firewall Pro, Online Armor Free provides both a firewall and a whitelist approach to program security for Windows NT, 2000, and XP. It does not show pop-ups for many known good programs, and it scans all your installed programs when it first runs so you can quickly tell it what to do with apps it doesn't know about.
When it does alert you to a new, unknown program, Online Armor's popups are informative but generally somewhat harder to decipher than those from Comodo. However, Online Armor goes beyond Comodo with a 'Safer' mode that allows apps to run, but with stripped-down privileges. Safer mode can work well for at-risk applications like Web browsers or e-mail programs, as it pulls administrator rights from such apps and prevents them from making deep system changes.
Online Armor Free has a learning mode, but you'll have to manually check for program updates with the free version. A $40/year paid option adds automatic updates along with online banking protection and other features.
If you're happy with your firewall and just want a dedicated whitelisting security program, System Safety Monitor Free Edition makes for both a quick download (3.25MB) and a quick installation under Windows XP, 2000, 98, and Me. You can set an advanced level of rules for what any given program can or can't do on your system. On the downside, you'll get an alert for almost every program, including common Web browsers, and the information in the pop-ups can be hard to figure out for nonexperts. It's easy to quickly change a mistaken decision, though.
Finally, if you want to access a whitelist with minimal impact, the Fileadvisor Windows Explorer extension, from Bit9 adds a right-click option to check any given file or program against the company's own online whitelist. You'll need to register with the site to get search results (which display in your browser), but since it doesn't block anything, you don't run any risk by using it.
For other free whitelist download recommendations, head to posts on the Wilders Security Forums and CastleCops, two excellent if somewhat technical security resources.
As these apps show, whitelist security may be a tool for techies today. But soon it'll be de rigeur in the battle against malware.