Well goodness me, gentlemen. It wasn't what any of you said wot done it (and even less what ASUS contributed) but rather that you kept saying
something which kept
me trying.
I had gone through safe mode on several occasions to command a return to different restore points - one of them in 2010, fully five months before the upset - all to no avail. I had tried reconfiguring to the last functional setup on two occasions, again to no avail. I had clicked on every hopeful looking bookmark for online help sites and scoured them for promising advice, but without success.
Then a couple of hours ago one of those help sites said "go to a restore point using this link". I gloomily did so and got completely different dialogue boxes displayed from what I had seen on previous attempts so I kept going. I picked a restore point at the end of February 2011 - and I'm convinced that I had tried that very point before - and it worked! I had a couple of hiccups which required reboots but I've got there!
Thank you all again!
PS I still have one issue unresolved on this newer machine but I tried you all on that about a year ago so I'll give you all a bit more of a rest before I bring it up again.......