I recently re-installed windows XP over the top of the original installation, to preserve my settings, files and programmes.
All worked fine, but now Windows Update will not work.
It did initially install 4 updates including 3.1 updater before it asked for a re-start, which I did.
It identifies that there are 80-odd updates awaiting installation, but if I choose express or custom install it churns away for a minute or so, then quickly finished, showing all updates have not been installed. I have tried doing update manually, using Microsoft update, etc but get the same result every time.
I seemed to recall that having time and date out of synch can lead to problems, but have checked that all the settings are correct for time, region, etc.