Phew, John/jbirkett's method just worked for me. Thank you, John! I'm running Windows XP Pro, SP2 with all the latest updates.
For what it's worth, here's a small detail that might be helpful to others doing this the first time:
After the bootcfg had scanned my drives (took quite awhile), it asked me if I wanted to add "D:\WINDOWS" (the recovery disk, I assume). I answered "Y", then just pressed enter on the following two prompts. When it asked the same thing about "C:\WINDOWS", I just pressed enter. I assume this counted as a "Yes", but I didn't get the two prompts I'd gotten previously.
When I rebooted, I was asked to choose which operating system to boot. I apparently had two options: a blank line(!), or Windows XP Professional. I chose Windows, and everything booted normally.
When I checked the boot.ini file, I saw two entries at the bottom, in the operating systems section. One of these had "" where the other had "Windows XP Professional . . ."
So I deleted the "" line from the boot.ini file, left everything else alone, saved, rebooted. Everything was back to normal.
Not exactly sure exactly what I should have done (No to D:\WINDOWS question, Yes on C:\WINDOWS question?) . . . but, thankfully, it doesn't really matter at this point.
