. . . if you find your PC beset by a really nasty trojan.
1. McAfee Rootkit Detective. Before you can solve the problem, you need to identify it. Rootkit Detective did that very well. (Google helped, too, as usual.)
These three pieces of software each fixed part of the problem:
2. The Avenger.
Finally, at a crucial point I couldn't do anything because the machine was booting into CHKDSK and then repeatedly hanging. Google turned up advice that I needed to boot from CD and then run CHKDSK /F. Several "boot" disks I had failed to work. But Dell's Reinstallation CD worked swell. Many people on the Net recommend to always have the manufacturer's OS reinstallation CD handy.