Which HP model is it? Some of the older Pavilions are not capable of disabling the onboard video...others are. Go here for <a href="http://www.hp.com/cposupport/software.html" target="_blank">HP's Drivers & Downloads</a>. Enter your model number and follow the bouncing ball. Hopefully, you can find what you're looking for.
Example: <a href="http://www.hp.com/cposupport/personal_computing/support_doc/bph04816.html" target="_blank">HP Pavilion PC - Motherboard Layout and Information: ASUS P2B-VE</a>. It does show a jumper for disabling the onboard video.
Let us know how it goes...Good Luck!