Have you installed the latest drivers for your card? Every time you install drivers, make sure that you follow the installation instructions to the letter—there are no shortcuts to good sound. Electro-magnetic interference is possible within your cabinet, too. Make sure that the sound card is not too close to other cards. Have you connected any hardware you don’t use? Every device reserves some resources for itself, so make sure you remove any devices you don’t use.
If you don’t have a microphone or a device connected to your Line-in jack, make sure that both the Microphone and Line-in are muted—any unused input is a potential noisemaker. To do this, double-click on the Volume icon in the System tray and check Mute for both. If you don’t see a volume slider for either, go to Options > Properties and in the dialog, check the mute boxes for Microphone and Line-in.