Here is a link to Creative's website. It should help you install your card:
Sorry, the link only takes you to Creative's website. So, go to the "Knowledge Base" area of the site. There you will find help for installing your card. Simply type "install soundblaster sound card" in the "search for keyword" section; item #2 in the results area should help you.
It sounds to me (no pun intended) like you didn't remove the drivers for the on-board sound prior to installing the card.