It is important that you have a Professional Plumber look at your piping. If the vent is blocked, or the drain is installed incorrectly, sewer gas can enter your living space. Besides being a bad smell, sewer gas can be explosive and/or toxic.
The end of the vent pipe must be out of doors, not in the attic. It should be visible unless your house has a flat roof. When your neighborhood is really quiet outside, run a empty load through the washer with the highest amount of water possible (large load). When the washer is about to start draining, go outside your house and listen for the sound of the water draining. The sound should be coming from the vent. If you don't hear anything, you may have a serious problem.
Too bad i'm so far away, I wish I could be there to help you.
Here is a good resource for information about sewer gas: http://www.atsdr.cdc.gov/hac/pha/pha.asp?docid=914&pg=1