all drain pipes are joined together so you need to find the source of the smell,, ie the bath room or laundry maybe kitchen,, then work from there ,, do you have a garbage disposal?? that is probably the cuase right there!! run the faucets for a couple of minutes after you unplug a drain,, this will wash out any remaining rsidues,,,
You need to remove the p-trap and clean it out with some comet or ajax and an old toothbrush. do this in another sink. http://video.google.com/videoplay?docid=1853816673809123811
After you clean it run 1/2 cup baking soda and 1 cup white vinegar down it once a month to keep it smelling clean. Let the mixture sit for about 30 minutes or so.
Poor some Bleech down the dains of all your downspots, Run some water through. If you can still smell it after running water, mix pine soil and little water and pour some into each drain hole. This should do it.