Thanks all! Actually, I've decided to reformat, reinstall XP and download the full SP3 volume, as this will leave the hard drive clean and hopefully pre-empt some of the upgrade nasties mentioned in Microsoft's TechNet Forum. So, now, Avast and other anti-malware won't even be installed when I download SP3.
If I did upgrade from SP2 to SP3 as originally intended, I would certainly follow the advice from this forum. I would disable On-Access Protection and other anti-malware before beginning to download SP3. I don't think I have been using the Self Defense Module. It doesn't appear in the Menu of the Simple User Interface. As nobody has mentioned Windows Firewall, I will keep it enabled and install my 3rd party firewall later - unless any of you advise otherwise.
It has been gratifying to read a number of prompt, informed replies confirming the value of this forum. I hope that your replies help other readers.
Kind Regards, Phil