There are a couple ways of doing this. The first way is to right click My Computer and click on Properties.  Then click on the Hardware tab and click on Device Manager.  Once in the Device Manager section, expand the link by Sound, Video & Game controllers.  Here you should see the type of sound card you have installed in your machine.

The second way to find out what kind of sound card you have is to click on the Start button, then click on Run and once the Run box opens, type DXDIAG. Now click on the Sound tab up at the top and the name of your sound card will be displayed.