Your HP Pavillion desktop has a 250 Watt power supply. The components in that system are using about 220 Watts. That means you have 29 Watts to play with at MOST! Let me put that into perspective. There isn't even a SINGLE graphics card by Nvidia that you can safely put into that system. Even their oldest card, the GeForce 5600 requires too much power to be able to add into that system. The only cards you can even think about adding are from ATI from 13 years ago. Like this one. http://www.amazon.com/ATI-Technologies-Radeon-Adapter-100-437009/dp/B0000T70TK/ref=sr_1_2?ie=UTF8&qid=1359774795&sr=8-2&keywords=radeon 9600 And even this, in my opinion, is a dangerous idea. If it even uses a couple extra Watts, you might have a power-shortage shutdown and you could end up breaking your computer. And to add insult to injury, I don't think it would even benefit you very much to add such an old card to your system. I think it's time to face facts and admit that, unless you're willing to buy that old PC another power supply on top of another graphics card, you can't upgrade it.