LL,
This is almost certainly an issue (which I have raised several times before) with the Version of PCI slots on the PC and the Version of PCI that the new card requires - see my post in the FAQ sticky on "adding a PCI card to your oldish PC".
I bet that if you dig a bit you will find the version of PCI that the card needs is PCI 2.2, and your PC slots are 2.1 compliant.
If you have a detailed product ID and serial number for the Pavilion you can probably find out the spec. from the the HP website -
www.hp.co.uk - or you can use the on-line detection tool. You will have to dig around to find the detailed specs. for your particular product, but it should be there.
SD