Yes. Your compaq case can work with most atx power supplies. I have an old antec 350w in mine. You may have a lock to undo when you slide the old ps out. Mine had to be pressed a certain way to pop it loose. All you have to do is check the position of the screw holes in back; some old compaq's had a custom square ps where all the screws lined up in a perfect square; the newer ones have the holes offset slightly and this is what you need to look for to fit the newer standard atx ps.