I found the problem! I'm sorry to have caused so much trouble and the fault should have been more obvious to me
. On the 2nd. PC I had thought I had deactivated the AVG firewall and virus protection, but had only deactivated the AVG control center. This suddenly occurred to me as I realised that the "broken" PC was still receiving AVG updates. I deinstalled fully the AVG software, and the PC immediately gained full Internet access. I then reinstalled the free AVG virus protection and it still works fully (I will have to work out how to reinstall the licensed full AVG portection including firewall - presently using Windows firewall with AVG limited to virus protection). Note the same AVG program ran and runs on the 1st. PC without any problems.
Again, sorry for all the work caused - many thanks to all but especially to Mr. Chris.
swissbill