Every PC w/ a mid case that I've had has had interchangable power supplies....unfortunatly I don't know if Compaq has anything proprietary.

Rip it out and go to a computer store and ask just to be sure. (or compare it w/ another computer) But i think you'll be fine.

Also, upgrade some since power supplies are so cheap...it'll be good to have more power if he starts adding in more drives etc.