I searched the archive, but didn't find any pertinent results, so please, don't wig out because I'm starting a climate control post.

Here's the deal. It was 10 degrees here (in boston)yesterday, and I drove my car to work. the car hadn't run for about a week, but started and ran fine. the temperature went up in the engine, according to the gauge, but the climate controll kept blowing out cold air. even when i tried to override it and put it in manual it never got warm. I was thinking maybe it was the temp sensor in the cabin, but the fan was racing to try to warm the car up, so I doubt it is that. From what I gather it could be the heater core, but the car is only a few years old, so I'm really hoping that isn't the case. If anyone has any experience or solutions to this problem they would be greatly appreciated.

the car is an '03 A4 1.8t