Generally speaking, the preferred method of doing window cutouts is a dremel and reinforced cutting discs (they last a lot longer than the normal discs), and small files/sandpaper to clean up the edges. If you want to cut a circular hole, another viable tool is a holesaw that's designed for use on metal (a holesaw is a drillbit that's designed to cut large diameter holes in materials). I'm also fond of a metal nibbler as long as the metal isn't steel/too thick (a nibbler is sort of like a pair of pliers in use...everytime you squeeze the handles, it makes a square-rectangular cut in the material...removes between 1/8" to 1/4" material with each snip).
Nibbler (i purchased mine from a local computer shop...just posted the link so you would have an idea of what i was talking about
