Had another play and found that I can reinstate the video driver and this is not the problem - normal screen resolution etc. restored. It seems it is just the CPU to AGP bridge driver, viaagp1.sys that gives the problem. Getting rid of this and allowing windows to replace it with its own (?) version viaagp.sys seemed to do the trick. Shan't be playing any games so the video is now good enough.
Thanks, Giles
