Oh - sounds bad. I changed all my relevant passwords (from a clean PC) when I realised there was problem (and have checked that nothing untoward has been happening with my financial accounts).
I appreciate your help and the time you have put into it. I'm guessing that (as it seems to be in quite a mess) it won't be cheap to get professional repair so it might make more sense for me to scrap it and buy another. Although I might be tempted to try a "reformat and re-installation" - but I'm assuming that's relatively straighforward and that I can get advice on that from an ordinary "how to" site.
Thanks again anyway.