NWFusion informed me of an interesting Earthlink.net press release. Earthlink reported the results of their customers running Webroot's Spy Audit program. This is a Windows executable which a user must download and run. Earthlink offers their own download, elsypaudit-i386-windows-all-2004.0.133.0.0.10.exe, which may be the same program, although the file sizes are fairly different.
Looking through strings output, I found a reference to http://spyauditresults.earthlink.net/index.php, which appears to be the results page once a scan is done.
I usually use Spybot Search & Destroy on Windows PC used by friends and family. I also used the Microsoft Update CD-ROM on systems with only dial-up modems.
The Earthlink study found an average of 28 instances of spyware per audited host. They also found over 184,000 installations of "system monitors," which Earthlink defines as programs that "can capture virtually everything you do on your computer, from keystrokes, emails, and chat room dialogue to which sites you visit and which programs you run." That sounds like Earthlink identified over 184,000 0wn3d Windows systems.