AFAIK, all HP computers contain "Proprietary Features" that make their power supplies non-compatible with ATX (industry standard) motherboards, and their power switch connections from the case to the motherboard also differ, so you can probably salvage the RAM, if it's fast enough for a modern system, the video card, if it has a separate one, and if it's not too outdated, possibly the display, and the storage devices.
Gateway sell a few systems based on standard parts, as does Dell, but in the latter case, it's a case of VERY few. Almost any major brand name you would be familiar with is probably proprietary.
