Vista to XP downgrade

Question

I got this Dell laptop loaded with MS Vista. My company's softphone software doesn't work with Vista so I had to do a clean install of XP Pro. Everything worked fine except the drivers for everything will not work. I went to the Dell site and DL'ed the correct drivers I needed, installed them in the laptop and nothing. The drivers say they install correctly and all that but they still don't work. Does anyone know if Dell does something to the bios? Or if there is a problem to that extent? Anyone who knows anything about Vista and this problem please help me out. Thanks a lot.