If cold weather doesn't stop you from exercising outdoors, then you're going to need some pants that both insulate to keep you warm, and wick away moisture to keep you dry. Check out the Cold Weather Pants from Nike ($65).
They're made with dri-FIT fabric, which is high-performance microfiber polyester. This material pulls sweat off your body and into the fabric's surface, where it evaporates leaving skin dry and keeping you warm. The backs of the legs have terry cloth inserts so you'll be able to move freely (I hate when the material bunches up behind my knees). These pants will help you stay warm and comfy.
Not only can you use them for exercising in the cool weather, but you can also layer them underneath snow pants if you ski, ride or snowshoe. They come in Black, Anthracite (gray), and Sable Green. Order them from Nike.com.