I know that rig is really outdated but are you yet within warranty? Just asking in case you blow that machine due to it being an old unit...? To save you the fireworks show inside(and outside) your case you may want to get yourself a new 450W psu.
I'd ask you to look into the Silverstone ST45SF-G unit. Its small, quiet and also is gold rated so you can use it in other builds - and ofc if you'd want to go SFF you can retain the PSU.
You could run off the unit as is but I can't ask you to open up the psu, inspect whats inside and slice open a capacitor to look at its innards. In fact a kill-a-watt meter is terribly misleading as well. If you didn't know a kill-a-watt meter is a device that is connected in-between your psu power plug and the main power line. The power drawn by your machine is shown on the kill-a-watt meter on its display.
johnnyguru ,the guru in power supply testing and reviews, has mentioned this a couple of times on various forums.
Can it run - Yes
Can it run without making you worry - No
The whole point of this thread is to give you sound advice right? I've dealt with old HP machines and their psu units are far from reliable once you add aftermarket hardware.
My 2 cents.