you need to determine if the motherboard is 100% ATX compatible.
Open it up and look for teh 20-pin connector on the motherboard. IF that's there, then it's likely that the Opus will work.
The only way to be 100% certain is verify the pinout of the HP PSU is the same as AT standard.
You can also see if the existing PSU is ATX comatible. There shoudl be a label on the side that will give you that information.
I'm pretty sure that the Opus will work for you. But you still need to do the leg work so you can be 100% certain.