Finally figured out what the problem was. Thanks for the response, hirenk; here's the short version of the story.

I sometimes do installs off a copy of the SQL CD that I keep on a hard disk. Somehow, a DLL on that copy got updated (ssnetlib.dll), so that it wasn't compatible with the base install of SQL.

My theory is that when I patched the machine where the copy of the CD is, the patch found not only the INSTALLED ssnetlib.dll, but also the version residing in the CD image.

Thanks again!

TC