Right Click on an open spot on your desktop.
If your Operating System is Windows XP, choose Properties from the menu box that pops up when you right click.
A new window will open up. Click the Settings tab at the top right.
On this menu you should see a slider bar near the bottom left under "Screen Resolution". Left Click and hold the mouse button on the grey button and move it on the slider bar over to the right one spot. Then click the Apply button on the bottom right of the box. The screen may flicker for a second and your desktop will change size. If the screen covers the whole monitor and the black bars are gone, that may be the correct resolution. Click OK on the box that has a timer and is asking if you want to keep the screen that size. If the screen does not look correct, click No and you may need to slide the button one more spot over and try the next resolution and Apply until it looks correct.
Under the slider bar as you move it over a line at a time will be 2 numbers like this 1024 X 800, and when you move the bar over the numbers will change. I believe the optimal resolution setting numbers for the 20" monitor you have should be 1680 X 1050 for the widescreen. You may want to try that setting. If it looks too small, try sliding the bar to the left and try each resolution setting.
Here's a screetshot of what the Box with the slider bar looks like: http://technophobik.co.uk/vxp_control_panel/f007_settings.asp
If you have Windows Vista, (it is similiar to XP):
Click the Start Button and click Control Panel in the Start Menu
Click Appearance and Personalization in the new window that opens up.
Click Personalization, then click Display Settings at the bottom
A new box will pop up with the grey button on a slider bar near the bottom left under "Resolution" Slide the button to the right to the next line one spot over on the slider bar and then click the Apply button. The screen may flicker and change the size of your desktop. Click OK if the screen looks correct or click No if it doesn't look correct and try sliding the button over one more line.