Let's just be sure what it is you're doing first, shall we? Make sure you're doing it this way.
Shut down all programs, or use the 'Show Desktop' icon in the 'Quick Launch' area of the Toolbar.
Right-click on an empty spot on your Desktop display and choose 'Properties' from the pop-up menu.
Click on the 'Settings' tab and try to move the slider to change the desktop resolution.
Is that what you're doing and does it work? If not, on the Settings Tab you should have an image of a computer monitor with details printed underneath indicating type of monitor and display card. what does it say?