You may achieve it giving the Spot Light a texture.
Add a Spot Light (Shift+A-->Lamp-->Spot), go to Lamp Data header and set up the settings depending on your needs.
Prepare your texture (you may find similar one in the net).
With a lamp still selected go to the Texture header, click a little bulb icon, add your texture and set the mapping coordinates to View.
You may achieve it adding two Spot Lights with different settings.
Play with the Strenght, Size and Blend settings for different lamps. When finished you may parent one lamp to another with Ctrl+P to make them move together.
EDIT: This method works well in Blender Render also.