You need to install your new sound card's driver - didn't you get a driver CD with it? (probably not if you bought the OEM version).
You can get the driver from the sound card manufacturer's website, in the support or downloads section.
You will also need to disable the integrated sound adapter (if the board has one) in BIOS Setup, otherwise there will be a driver conflict between the two sound drivers which will stop sound from working. Your system might disable integrated sound automatically when your new sound card is detected, but best to check anyway.