Just Like BenJeremy said, just use a default screen as a guide:
Open up your desired background, and set the image size to 640 x 480:
Then open up an already completed screen to use as your guide:
Drag the completed screen into your new background (left-click and drag):
Now you have the already completed screen over top your new background.
Select the "Rectangle" tool:
Using the "Rectangle" tool, draw new squares, using the completed screen's squares as your guide:
Delete the completed screen, and your squares will be in perfect position for your new screen.
Use PhotoShop's editing features to edit color, opacity, bevel and emboss, etc.
To add text, just simply place the text in the spots you want to. Save your completed pic as a .PNG named "screen" .
