Here's what happened. I did a search on Dogpile for "winter solstice". Clicked on a very innocent looking link that came up from the search (I don't remember the URL or verbiage). I was INSTANTLY alerted by Webroot Security Essentials that I was infected with a malware bundle consisting of:
these "very highly severe" Trojan horses:
1. Trojan-Phisher-Snifula (Unique Code 6FANL78I),
2. Exploit-Java (Unique Code DSHQ6ADG), and
3. Rootkit.Gen (Unique Code IQ1LCWD7),
and these "highest severity" viruses:
1. Mal/Bredo-B,
2. Mal/Hiloti-A,
3. Mal/JSRedir-C,
4. Mal/ObfJS-CM,
5. Mal/ObfJS-H,
6. Mal/ObfJS-X,
7. Troj/Istbar-DQ,
8. Troj/PDFEx-CM,
9. Troj/PDFJs-ER,
10. Troj/PDFJs-FZ, and
11. Troj/PDFJs-GE
which Webroot had already auto-quarantined. A subsequent Webroot sweep and running some utilities Webroot sent me eradicated them all except Rootkit.Gen, which Webroot tells me I have to pay them $100 to get rid of (which I actually don't blame them for; it looks like it'll be a real bear to get rid of).
Under the circumstances, it's hard to see how I could have avoided this infection, short of finding and installing some hitherto unknown super-powerful anti-malware, since Webroot was unable to block it.
Anyway, I've consulted a few special forums run by malware removal professionals. Although they doubtless have the most experience in such matters, I think they're biased in favor of trying to remove the rootkit, rather than just reformatting the disk because that's their hobby as well as how they earn a living (although, in fairness, they're not asking for any money). I understand the fascination of trying to remove it, but from the feedback I'm getting around the community, I'm actually leaning towards reformatting, because the consensus seems to be that you can never be completely certain you're not still infected otherwise.