Hi,
This morning (a cold morning, but not the coldest this winter) I turned on the cars heater, and after a short while, I noticed steam coming from under the hood near my windshield. (Outside the car). There was also an engine smell coming through the vents. I turned off the heater and continued driving. The steam stopped. The smell went away. Everything seemed fine. I kept an eye on the temperature gage. It never moved from mid way between cold and hot. (it's normal position). When I got to work, I again turned on the heater to see what would happen, and the steam and smell came back immediately.
Any thoughts? (1999 Nissan Altima)

Sounds like the coolant is leaking somewhere between the engine and the heater core.
Have you lost any coolant? Check the coolant reservoir and look under the car as well.