Well, I can now watch YouTube and similar without it freezing but not without a lot of hassle on the way!
I decided to do a repair install of XP Pro; this was quite straightforward except for hitting the old 'setup freezes at 34 mins remaining' problem. This took about three hours to resolve and was eventually sorted by removing all pci cards except graphics at which point setup continued. (For the benefit of others, it is caused by a hardware driver problem hence removing all unnecessary hardware).
I then had to refit the expansion cards and install related software. This was fairly trouble-free except the system would not 'see' the graphics card. This problem was finally resolved by re-installing motherboard chipset drivers from Gigabyte's site. The m/b disc which I had wouldn't work because the system thought the drivers were already installed.
Finally at 2130 everything was up and running as it should (including YouTube videos) having started at 1400. Still. I was in a cool room and not out in the 30+ degrees of East Anglian sun!
Throughout all this, 'Google was my friend' and it directed me to the many other souls who have had similar problems to me. I commend it to you all but you probably know that anyway!
Thanks to all who offered suggestions.