This is a serious problem. I formatted a friend's computer (it was an HP) and I never did get the drivers for it. They just weren't listed on HP's site. No ethernet, sound, or USB. We had to do the Recovery discs (six discs worth of spyware and useless programs), restoring it to it's original, lag-tastic and slow state.
But to answer your question, hit
http://welcome.hp.com/country/us/en/support.html
and do a search for the drivers you need.