Waaaaaait a minute - the BIOS is the BIOS, and as such has nothing to do with your on-board sound chip. (Ok, you can activate/deactivate it using a BIOS setting, but that's about it).
Didn't the mainboard come with a CD that contained chipset drivers, drivers for the onboard IDE controller, drivers for the onboard USB functionality - and for the onboard soundchip?
Let me check something... *goes browsing*
A-ha. I just saw that mainboard uses the ALi MAGIK 1 chipset and comes equipped with a C-Media 8738 onboard audio chip (optionally).
Check this page:
http://www.asus.it/support/download/...266-E&Type=All
You'll find links to both C-Media and non-C-Media onboard audio drivers... the ones for the non-C-Media version include a driver explicitly designed for WinXP, the ones for the C-Media Audio don't. In that case, however, I guess you can use the Windows 2000 driver, since both are based on the WDM (Windows Driver Model) architecture - unlike Win9X drivers, which are VxD drivers.
Good luck - sounds like you'll need it...