Somehow I got infected by XP Guardian 2010.
I'm not sure what it was, exactly, but it slipped onto my computer when I wasn't looking and lodged itself into my Windows Security Center.
Rather disturbingly, the infection seemed to occur within hours of upgrading that machine to Firefox 3.6. Firefox has been incredibly reliable for me over the past few years, so this must be a red herring. Still, it's the only significant change I can recall making to the machine in the last period of time leading up to the infection.
I fought with it for an hour or so, then bit the bullet and used the "Access IBM" button on the Thinkpad to completely restore the computer to the factory state.
Then I spent about 4 more hours running Windows Update, and re-installing the core applications that I really care about.
It was time, anyway; I hadn't rebuilt this computer in about 4 years.
I love this Thinkpad T43p; it's been one of the best computers I've ever used.
Now I'm back, running XP Service Pack 3, with Thunderbird and Google Chrome.