how do i change the screen brightness without using the fn key?

Hi,

1.     What is the make and model of you computer?

2.     Is it a Laptop or Desktop?

I would suggest you to refer the following steps and check if it helps.

a.     Click Start, click Control Panel.

b.     Choose "Appearance and Themes" when prompted in the "Category View" panel. If you're in "Classic View," choose the "Display" icon.

c.     Select the "Settings" tab from the Display Properties window.

d.     Select the "Advanced" button on the bottom of the window. This will take you to your monitor's properties.

e.     Choose the "Color" tab.

f.      Select the brightness scale and push or pull it until you are satisfied with the brightness of your screen.

g.     Press "OK" to set your brightness.

More information refer:

Adjust your monitor's brightness and contrast: http://windows.microsoft.com/en-US/windows-vista/Adjust-your-monitors-brightness-and-contrast
Note: Applicable for Windows XP as well.