HP uses ATX power standard, but they usually have smaller power supply housings. The two most common are SFX (smaller in every direction) and PS-III (looks full sized from the rear, but is shorter inside). A full sized power supply sometimes fits where a PS-III style was mounted, but often using the longer supply will cause it to hit the back of the CD drives.
If you're lucky you have the full sized unit. Small sized power supplies normally don't come in sizes above 250W. Either way, it's on the ATX power standard so you won't blow anything.
Dell was the company that rewired their connectors to blow your components if you tried switching to other brands.
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