Finally got things working again. Thanks for the suggestions chaps, but in the end it turned out to be something quite different. I haven't found out what caused that strange .INI file entry, but just after my last post everything keeled over with a "Not Enough Memory" error. Cutting a loooong story short, the culprit turned out to be Norton System Works 2004. Uninstall, followed by a complete manual removal of all the bits Uninstall didn't, followed by a re-install, seems to have solved the "out of memory" thing. Three cheers for Selective Start-Up.
Sorry williamsg, back-ups?? One only does back-ups after a fright like that! Like wot I have just done!