My wife and I bought this house in Aug of 2005. Oct of 2005 the vents started to smell. Bad, after a few days it was intolerable. We had to shut off the heat. It was coming from the heating vents and the returns. Couldn't figure out what the problem was so we had the ducts cleaned. ~$450. That did the trick, until now. Same thing again. Around the same time. We have central air and all though the summer we didn't have a problem. But starting two days ago it came back. I can't afford to spend $450 every year, this close to theholidays no less ;)
Any ideas on what might be causing this, and can I fix it myself?
I thank you for your time.

Have you had the ducts checked for mold?

Just did and the guy couldn't find any, but he did find something. It was in fact mice. There were two in one of the returns. Would explain why it happens around this time of the year. So we are going to mice proof the garage and house. Hopefully no more will get in. In the summer we had to work on some things in the garage, thus leaving the garage open. I bet they got in then.