Well cost wise, that would probably benefit from an inverter.
I know HP and Compaq are the same, but I cannot find any mobo pics of a Compaq DC7800, but I found some HP DC7800. Not sure if they are the same.
From the looks of that, it has a miniHP ATX connector on it, that a regular ATX PSU can be adapted to fit with some wire cutting and soldering.
Can you open up the PC and take some PICs of the mainboard?