The drivers for VISTA and WXP are different, and not always will you find XP drivers for a VISTA machine...
You download the drivers from the computer maker, if there is one.
Otherwise, you download them from the maker of the component... You get the part numbers and description information off each card, then do a Gurgle search to find it...
Or list them here, and a lot of TechSpot posters will help you find them.
List the motherboard brand and ID digits... to find the onboard Audio.
Or list the audio card to begin seeking the audio drivers for the card.
You will again find the video graphics from the video card ID, or if onboard, again from the motherboard maker.
You can likely find the GeForce 6 drivers at the NVidia site.
Audio from the audio card, or the ID digits on the largest chip of the audio card.
You might even have to change the Chipset as it may not be the same for both VISTA and WXP. There could be other differences, so check them all... Audio, BIOS, Chipset, Video Graphics... downgrades are a lot more difficult that upgrades.