|Did you buy some sort of package deal? Speakers plus a sound card?|
To disable the onboard sound, you have to press DEL when you 1st bootup, then go into the BIOS. Look under the "Advanced Chipset Features" menu for "Onboard Device" & change the "Audio Controller" setting from Enabled to Disabled. Save the settings, exit & shutdown.
Unplug the power cord, open the case & install the PCI soundcard in any available PCI slot. Screw it in place, close up the case, plug in the power, bootup & install the drivers from the CD that came with the card.