At last I have fixed our dell dimensions 2400 and it now runs well with windows 7. I installed a number of drivers
but none of them seemed to make a difference but then the last one which was R47822 did. Now I can change the resolution because there are a number of options to choose from and the graphics are as sharp and clear just like my new laptop with windows 7.
I went to dell web site, drivers & downloads and typed in dell dimensions 2400 this then gave a list of all the drivers that a 2400 needed with the option to download them.
So all those of you out there who have old pc and want to upgrade to windows 7, theres hope yet.
Thank you to all those who have posted regarding dell dimension 2400.