More info about your sound card, motherboard and windows version would have helped.
Perhaps you have the wrong sound card driver. If it is a sound card built-in to the motherboard, then you need to check it on the manufacturers web site as they sometimes have several possible drivers depending on the exact motherboard revision.
You don't say what version of windows you are using, perhaps you need to install servicepacks which may fix problems if using the default Microsoft drivers.
Get the latest drivers from the sound card manufacturer and if all that does not work, try with a different sound card.