I don't think its Micro ATX, looks like Flex ATX? Not sure if that is compatible with Mini ITX although it appears the size is similiar. You can read about your power supply here:
HP Slimline Desktop / Tower Replacement Power Supply HP P/N:5188-7520
Newegg sells
Mini ITX it looks like the dimensions are close, not sure if you want to risk it or try and find an OEM part. Good Luck!