Hello Blackbird, thanks for the information.
By the way, I tried Apple Safari and rejected it because it is very difficult (under Windows) to use search engines other than Google, Bing or Yahoo - and I don't (and won't) use any of them.
Yes, it is strange that blocking certain URL's causes the programme to crash (the dashes in my last message were originally "ctrl-alt-del" (with <-> - I suppose the forum software thought they were HTML).
My firewall is Sunbelt (Kerio), I only have an anti-virus AVG on one (different) computer in my network for testing purposes (as a professional I don't ever do what one shouldn't and I have never had any virus).
There are no multiple user accounts on any machine (except 'administrator') and everything is always installed for only the current user.
After the Opera crashes it was necessary to use ASR to restore as no restauration points worked. So I lost any later history.
From memory it came from clicking on a link within Opera which led to an archived version page (I downloaded it a while ago).
I have only ever had the v. 11.10 installation program - it downloaded v. 20 from Internet during the installation without asking. I only realised what had happened when I saw it installed in the Control Panel -> Add/remove programs.