Hello again,
OK, well I did what you (Asyn) suggested, and it did seem like Avast had installed OK. It didn't ask me for an email address or anything, so I suppose I should have known there was something wrong, but for a short time the Active Protection seemed to be on.
After a restart, however, it's not working again. The File, Mail and Web Shields are off, the Update page says the virus definitions current version is unknown and the connection not established. Registration says the program will expire on 10/5/2017, however, so I appear to have been given the full year of access (if only it worked).
I do have an ongoing issue with this PC, which is that it won't shut down properly, it gets half way there and then hangs with a blue screen (no error codes, though), so I have to hold the power button down to switch it off.
Is it possible that Avast needs a perfect shut-down to be able to start itself again properly? That doesn't really make sense, as any restart after a power outage would leave the PC unprotected.
Anyway, whatever the issue, it's not working for me, so unless someone can come up with a good suggestion I think I'll be forced to give up and go to another anti-virus...
Piers.