OK - I got it fixed. After pulling out what little hair I had left, I removed an old version of Malware-Bytes and then downloaded a new version, saving the download file to the desktop (thinking it would be in the Desktop toolbar when I got the chance to install it). When I came back later to install it, the desktop was back to the way it should be! I deleted the desktop toolbar and the new "old" desktop has been in place for the last 2 1/2 days. Strange.....