I am supprised you didnt get the Hardware Application Layer - ( HAL ) Blue Screen of Death (BSOD) when installing a new motherboard with a different chipset...
You have gone from a ATI one to a Nvidia one with out reinstalling.... erm ok
Buy hey ho i guess you havent got a Windows XP Home OEM cd which you could used with your HP Windows Code.....
I would suggest you buy another hard disk and borrow a windows disk from some one to reload windows and start affresh, and use the code on the box, u will need to ring windows activation and explain the situation and they will activate it....
If you cant do this removal all old drivers including sound, network and motherboard
re install new motherboard drivers, having the latest from the www.nvidia.com will help
then install the sound card drivers, its a realtek hd - get these from the evga.com website
goodluck