I'd assumed that monitors displayed what the graphics processor/card told it to so I obviously know less than the little I thought I knew about monitors/graphics/drivers - doh!
I upgraded to 2 X 2 Gb memory from 2 X 1 Gb, changed to a wireless mouse and added an external USB drive for backups, apart from that the PC is as I bought it.
Sorry about my confusion with the name of the graphics, it should be an ATI X1300 Pro 256. I dug out a Maxdriver report from before I upgraded from Vista to Windows 7, which reported on both an ATI X1300 Pro and an nVIDIA GeForce 6150. Since drivers are something I'm unsure of I waited until after the Windows 7 installation and as there were no problems, left well enough alone.
I downloaded and tried installing R129368 again, but it's a chained process and at some point I got the message that it needed to be run in Administrator mode. I tried guessing which part of the installation it was sticking on and tried running them directly as Administrator butstill got the same message.
I found an update to the X1300 Pro 256 driver here -
which is R160911. I downloaded it and tried running it but it just seemed to disappear, with no messages, no errors and no installation - nothing.
I opened the PC and checked that there's an nVIDIA chip on it that I think must be for the graphics with a number NV06562036 0651A3, GS 2678.1 written on it.
Belarc advisor didn't report any graphics details, only the monitor, but there is an nVIDIA nForce Serial ATA controller.
I've also just found R160915 on a Dell Dimension E521 site here -
this is a 64 bit Vista driver. I downloaded and installed it and after a reboot the display is now much better, the resolution has defaulted to 1680 X 1050, with 1280 X 800 and 1024 X 768 as alternatives. The default 1680 X 1050 works perfectly and the display is much clearer and finer, the warning message about damaging hardware has disappeared and my carefully crafted Excel workbook now displays exactly as I inteended - so I think it's a success.
If, at some point in the future, I decided to update the display would I need to upgrade the monitor or graphics card , or both, please. Or would it mean the system?
Thanks again for all your help, I think I was chasing my tail before your advice.