As best as I can remember, this is what I did:
I went into Control Panel>Folder Options>File Types. There I saw a bunch of different file types listed. File Folder and Folder were there, and since I wasn't sure which one I needed to fiddle with, I picked File Folder first, and changed the icon. This didn't do anything to change the folder icons anywhere, so I did a Google search which turned up something about how customized settings don't show up on XP unless you change the desktop appearance to Windows Classic style. I did that and restarted my computer.
Still no dice, so I just changed the appearance setting back to Windows XP style. But then I tried to open a folder and discovered the problem I'd created. Then I tried restoring the default folder options settings a few times, tried restarting, etc, but nothing helped. This isn't the end of the world, as I can still go into a folder and right click and hit Open, and that allows me to see all the contents. However, this is bugging me because I would prefer that the folders open properly and because I did something, and I want to know what it is.
Oh well, the search continues! Thanks for your help.
L.