You cannot download an audio controller since it's a piece of hardware.
The controller still exists inside your computer, but since it no longer has it's driver installed Windows does not know what it is -- it's just an "unknown device" (yellow question mark in Device Manager).
Install the Realtek driver and Device Manager should list it by it's proper name again.
The latest Realtek High Definition sound driver for the ASROCK 'ALIVE' series of motherboards is here (for Windows XP):
http://www.asrock.com/mb/download.as...&s=&T=D&ID=589