I finally bit the bullet and did a installation repair of the system. The PC works fine now, except for one fact - XP Pro cannot "see" the Radeon X800GT AGP card. According to the Hardware Device manager, I have no display adapter and the system refuses to install drivers downloaded from the AMD site. Even the installation software from AMD balks since it too cannot find the card. While this is going on Windows is using the standard vga driver.
I would have been inclined to think that the card might be faulty but.... This is a dual booting PC and the alternative OS is Lubuntu. The card is identified absolutely correctly in that system and the driver used is excellent.
I would be grateful for any help in getting Windows to recognize the video card.
I solved this. Somewhere in the Hardware Device listings was something called a Microsoft 3.5 AGP Filter abd a little further down a ViA AGP something or other. I uninstalled them and re-started. XP now said it had fouund new Hardware and offered to install, I declined (assuming that if the system could now "see" the graphics card, ATI should also do so) as I had already an ATI driver handy. Installed the ATI driver and it worked, so this machine is now 100%.
Edited by ptak30, 20 July 2014 - 04:07 PM.