Progress of sorts - I forgot that I had a DriverScanner. Ran this ad it found a number of drivers listed as out-of-date, three of which were graphics related: 1) the graphics driver; 2) NTATIvrv01; 3) SiS AGP driver (see attached PDF file).
As the problem appeared to be graphics card related, I installed the VGA driver first - this installed a Radeon 9600 driver that gave the same code 43 error. I then installed the SiS AGP driver, but on restarting the computer in normal mode the login screen was broken up into 'wiggling' horizontal lines.
When I restarted in safe mode and looked at the Device Manager. Neither Sis AGP driver was reported to be working normally, in addition so was the Radeon 9600 driver - in addition, for the first time since the problem occurred both the primary (digital) and secondary (analogue) drivers were present. Installing the NTativrv01 driver didn't appear to have any effect. I used the system restore to go back to the situation before installing driver updates and am back to square one
However, it would appear that the problem is not with the graphics card / driver, but with the AGP port driver. Since the update was only one version higher that the old driver.
Any idea what is causing the problem with the AGP driver and is there anything I can do to solve it? I'm at a loss as to what to do because if the issue is caused by the AGP bus I wonder whether buying a new graphics card will work.