Is that a 64 bit version of XP that you installed?

 

Some of the problems with downgrading is that the xp install disc's generic drivers may not cover all the latest hardware. If you find the drivers you need for your laptops basic features you may still have trouble with specific things on your model that the manufacturer typically supply the drivers for such as extra controls and shortcuts. Probably should have checked with HP prior to the downgrade. Can you re-install Vista? If you are not sure what drivers to install, I have used a bootable linux cd, preferably pclinuxos or mandriva for the control center, then you can go in to the hardware and see just what is reported. This will let you see, if it works, what hardware drivers were used to boot the pc. You can then sometimes find a working driver from the manufacurer of say the chipset, audio and video, etc.
This is how I've done it, maybe there is a better way. Did you look into dual booting?

 

Get us the DEV/VEN numbers from Device manager for the devices you need drivers for.

To open "Device Manager", right click on "My Computer" (Computer in Vista/Win7) select Properties, On the Hardware tab Select Device Manager, if you see any yellow question marks, right click on the device and select properties, on the Details tab select Hardware IDs copy the PCI/VEN and DEV numbers and post them on your next post.