the PSU is the power supply unit
if you open up your HP (i've worked on 6 different HP's in the past year, and they are all hell to get apart, good luck,
), and look at the top of the of the case where the power cord plugs in, there should be a box screwed into the chasis with power cords running out of it to your drives and your motherboard. Gently disconnect the power cords from the motherboard and your drives, unscrew the PSU, and remove it from the case, then go to best buy or compusa and buy a new one. 300 watts should be plenty for the box you have. And a word of caution: do NOT open the PSU yourself, there's a ton of electrodes and capacitors in there and if you touch the wrong thing you're gonna be in for quite a shock :P (pun intended) lol