Hi Caleb,
Thanks for your prompt reply. Perhaps I did more than just delete my keychains, because that didn't work. When I open Keychain Access now, the only one there is for my Airport base station, which is a System keychain. When I open Mail and click Get Mail, it asks for my password every time, even though I have checked to remember the password in Keychain. My external drive was mounted at the time, so maybe its keychain was deleted, too. I guess the worst case scenario is that I will have to reenter all my passwords.
Cheers, Peter