PROBLEM SOLVED!
I am pleased to report that avast! is now successfully installed and is automatically updating. My thanks to Technical, RejZoR and S.Z.Craftec for your advice in achieving this.
The problem all along was the presence of Norton AntiVirus and Norton Internet Security. The way of resolving it was simply to completely remove both applications, thus allowing avast! to work unimpeded. I had hoped to retain Norton Internet Security in a 'belt and braces' approach to preventing attack from the web, but accept that this probably isn't necessary.
Thanks again for all of your help.
Spud
ps: why am I a llama?
