Meant to add the following to my
tale of woe:
While I was sitting around, watching the reinstallation of WinXP proceed at its usual glacial pace, Marcia called downstairs in a panic for me to come up and help with her computer.
Suddenly, she couldn't get the quick-launch bar to function. She couldn't get into Mozilla that way, and when she tried to use the Start button, all that happened was the start menu flashed up and disappeared. I tried once or twice; same result.
Then I looked more closely at her keyboard, shooed a cat away from where it had been resting on the Escape key, and tried again.
Problem solved.