Hello Tech!
Thanks for that, I will try again with the Avast uninstaller, but I am not at all sure that it will make any difference as in the interest of trying to keep the post short there is something I didn't tell you, which is as follows.
As I had tried five or six times to install and get updates to work, I decided I had to get rid of anything that might be causing the problem. So now you will begin to see what a devoted fan of Avast I am, or maybe just think I am crazy.

I decided that I wasn't going to be beaten by this, so download yet another fresh copy of Avast 5 stored it on my external drive then reformatted my main C drive and did a clean install of XP. Then with only IE8 installed, no other programs at all installed, not even a firewall or anti-spyware installed the fresh copy of Avast 5 to it default settings, except I declined the Google Chrome option (I have tried Google Chrome and do not like it much).
Then with nothing else installed except IE8 and Avast 5 I registered and it updated the registration to Feb 2011, but again I did not see the definitions update process, so after a long enough wait I decided to do it manually and of course it just sat there as before switching the cannot 'connect message' on and off.
I am not able to try your suggestion right now but will do so when I have enough spare time to allow for it not working again.
All suggestions/advice appreciated, I really want Avast 5 and feel very frustrated by this.
Thanks
Jack