Computer wont allow change of display

[green777]

Thanks dabhand for all your help, ill have a go with that info shortly.tried that could not get any files to load also tried the monitor clicking on desk top got all the way to the monitor tab but still would not allow me to open the properties,got the right files at add but will not allow me to access them.When i checked my own computer went to display properties you get the 2 screens up and under that it says plug and play radeon etc, but when i try the display properties on the optiplex
there is no mention of the card details at all only default monitor,