To access your display properties – click on ‘Start’ (bottom left), then select ‘Control panel’. From the Control panel window, open the ‘Display’ icon. Select the ‘Settings tab’ if you then see more than one ‘display’ shown, you should ensure that there is a tick in the box against ‘Extend my windows desktop to this monitor’. You need to click ‘Apply’ and ‘OK’. You might have to select the relevant display from the drop down menu.
The above instructions are based on Windows XP.