ThinkComputers reviews (and of course installs) a window kit and an Appliqué from Crazy PC.  They did a very good job going over how to cut the hole and install the window.  Very good read if you're looking at putting a window in your box (like I am).

This window kit is a great project for the do-it-yourselfer kind of person. It doesn't require allot of tools to complete a project like this. Maybe an afternoon or so, a Dremel and masking tape. This mod is not a expensive project for the average person to complete. The 12"x12" window kit that you seen is this review sells for $17.95, and the appliqué's in many styles and shapes sell for $8.95.