A computer system, a digital camera and some local currency
Step 2: Remove your monitor
Remove your primary monitor and balance precariously in an unsuitable place.
Step 3: Take a photo
Now photograph the view that a user of the computer would see if they looked through the monitor. (The local currency can be added at this point for side-splitting hilarity)
This step needs to be done very carefully for maximum effect, sitting on the chair usually used at the computer when taking the photo ensures the angles are correct.
Step 4: Manipulate the image
Manipulate the colour, size and position of the image until it looks like the view through the monitor. A good program for this is the open-source program 'The Gimp'.
Now set the image as your desktop background, sit back and wait for the humour to happen.