The frequency had increased from once per day to 2-3 times per day, but I noticed that Internet Explorer was spontanteously popping open with "Windows Antivirus" ad windows, so I ran Spybot S&D 1.4. It caught some things, but nothing that looked definitely like the culprit. I immunized the system against some 12000 malware entities.
I then ran Ad-Aware. At one point during the scan, it bluescreened with a STOP error (no reboot). After I restarted and repeated this problem a few times, I realized that it was the SpyBot immunization. I ran it again, and it detected a checksum failure (indicative of tampering). I reinstalled, but did not immunize.
Then I ran Ad-Aware again. It bluescreened again, but not before I was able to nuke one item of adware. Finally, I ran VundoFix, and it removed several instances of Virtumundo.
Now the computer is a bit more stable, crashing only once a day, but I still have no idea what people are thinking when they send out this stuff. Not only does it hijack your computer - it has the same behavioral signature as the worst malicious viruses! They thwart detection by modifying scanners that would otherwise find them... and again, what are we supposed to think? "Wow, how clever. I could really use a Tronco Blargotron."