Thanks, @GarryP and @loggerjack, for your helpful responses.
loggerjack, I first followed your advice. Curiously, when attempting to add the path C:\Windows\Fonts - copying the path from the File Explorer - Scribus gives an error message saying the folder does not exist.
GarryP, I uninstalled the Raleway font and reinstalled, using the files you sent. That did the trick, and Scribus now recognizes the font.
If either of you (or anyone!) has a notion about why Scribus is not recognizing a folder that I can access through other programs, it would be interesting to hear.
Much obliged,
John