Problem solved. This post is to help anyone else who may run into this problem. The file \Windows\System32\ITSS.DLL is somehow involved in displaying .chm files. Mine somehow got deleted in the recent past, probably as part of a software uninstall. I restored the file from another computer and all now works properly. FWIW the only software that I have uninstalled recently in Norton Internet Security - hmmm? Thanks to all who responded.