Is there a screw holding it on at the top?  If so, it might be a "torx" (6 
point star) or "tamper-proof torx" (6 point star plus pin in centre) screw; 
Compaq loves 'em.

If it's just a metal casting you have to break out, you might be able to pry 
it off with a flathead screwdriver in one of the holes.