Right click anywhere on your desktop and select Properties from the drop down menu.
Select the Settings tab in top right corner of the window.
Move the slide bar in the "Screen Area" or "Desktop Area" box until it reads 800 by 600 pixels.
Click OK on the warning message. Your Done!
Note
Your monitor and display adapter determine whether you can change your screen resolution.
After you change the desktop area, click Test to determine if your monitor supports the selected resolution.