I had this same problem. I've got an HP Pavilion running Vista but maybe the issue is similar on either. Not sure if you've found a fix yet, but this worked for me.
Right Click on the Sound/Volume Icon by your clock on the taskbar.
Go to playback devices.
Right click on your speakers/Headphones and choose Properties.
Now, under the Levels tab on the top, mute the "PC Speaker" but leave the "Speakers" as they are.
My buzzing stopped immediately and the sound hasn't been affected otherwise. Hope this helps.