Have you tried System Restore to some time before the hardware crash,
Remove ZA (which version do you have?), put on AVG (I find AVG 7.5
"works")? No doubt you would have put on any backup if you had one?

No I didn't - I decided the missing folders for AVG and ZA were first
priority, then when I ran into reinstall problems I thought there
might be missing system files too so I reinstalled Windows - which of
course wiped out all the earlier Restore points. Catch 22.

I have since realized my 'normal' connection is also very slow (only 3
or 4K) and the cause of the additional slowing isn't Internet
connection but something else - no idea what yet. Wondering if a
registry patch of all the settings that might be affecting it would be
worth trying, but I don't know where to get it from. Any ideas please?