Paul, I found a solution thanks to Ken Camp at Experts Exchange (see Welcome to Experts Exchange). I think it's the same one you were trying to tell me but I didn't understand correctly. I found an RStudio folder at C:\Users\Owner\AppData\Local and C:\Users\Owner\AppData\Roaming (the AppData folder is hidden). I renamed these as RStudio_Old and was able to open RStudio using rstudio.exe in another directory. Opening RStudio created new folders of the same name in both Local and Roaming directories. The new RStudio folders have less files in them than the previous ones.
Is it OK to delete the old RStudio folders and the files they contain? Do you know what caused this problem to occur?