I found a couple of cross-references for the power supply for your Pavilion XT978.
This one and
this one are rated at 200 watts.
This one is rated at 230 watts. In either case, I'd say your power supply is way underrated for your needs. For a 2 GHz P4, I'd want at least a good, true 350 - 400 watt supply.
Is your machine still in warranty? If so, get help from HP, and insist that either they make it work or allow you to return it. Then, hang around the forums, learn what components will do what you want, and use the money to buy something more capable of running your games.
Sorry, but I don't have a lot of respect for department store computers for exactly the reason you've encountered. They're built just strong enough to light up a Windows screen when you take them out of the box, but when you really try to use them the way you want, you're in for disappointment. The power supply isn't the only place I've seen these things cheaped out.
Good luck.
