I've tried with no firewall and I've tried using Internet Explorer, Firefox and Chrome.
Perhaps try downloading and saving a copy of the installer to a different PC. (If you're on a 32-bit Windows system, try to download on a different 32-bit Windows system.) Copy the iTunesSetup.exe installer to an external hard-drive, or a large-enough thumb drive or burn it to a CD-R. Carry the installer back to your PC, copy the installer to your hard drive. (Don't run the install from the ExHD, thumb or CD-R.) Start the install by doubleclicking the iTunesSetup.exe file.
Thanks, sounds a good plan, although I don't have ready access to another Windows XP 32-bit system. I'll give it a go when I get the chance.
I have downloaded upgrades many times in the past with the same set up. Come to that, I download other files almost every day. I could understand if the installation was the problem but it seems truly bizarre that I cannot even download the installer!
Thi solution worked - thank you. However, it will be a real pain if I have to deal with every future update in this way. Incidentally, the laptop I downloaded on was using the same anti-virus and firewall software as I use, with the same relevant settings, so it could not have been anything to do with that!