you have to get into your BIOS
press delete as when the system starts (black screen) and u'll find urslef into blue screen , means BIOS settings
you'll find their in On-Board Peripehels seection, on-Board Audio, simple disable it.
btw, i didnt myself lked the oon-board audio that came with P4C800/P4P800 motherboards, it sucks, isnt it?