Try going to the Control Panel>Sounds and Audio DevicesJ>Click on the Audio tab and then on the pull-down menus, select the newly installed sound card as the default device to use.
You can go into the bios or in XP, go into the properties of my computer and then hardware>Device manager>expand the sound, video and game controlers and then select the onboard sound device, right-click on it and disable it. Do that for all the onboard sound card items listed.