i just fitted a new soundcard myself a few weeks back,upgrading from my previous onboard sound.all very easy.
every bios will be different in how it is layed out,but i dont think you should have to much difficulty,heres what i did using xp home and an ami bios.
check in control panel/performance and maintenance/system/hardware/device manager and look for "sound video or game controllers"
find your onboard device listed here,if it is then right click it and select uninstall.
shut down the computer.insert the new card and as soon as you turn the computer on,enter the bios straight away,dont let windows start.
click here for popular keys to press to enter the bios.it will also say on your screen something like"hit F2 or DEL to enter set up"
once in the bios disable your onboard device(mine was listed under the advanced/peripheral configuration menu)
as said above use the "up/down/left/right" arrow keys to navigate the screens and "enter" key to select an option,up down to change it and enter again to set it.
save your changes(will say at the top or bottom of the screen which key to press to save and exit)for me this was the F10 key.
boot into windows,it should find the device and prompt for drivers to be installed.
place your cdrom driver cd in the drive and point windows to this to install them.
its a five min job.