I'm sitting in the kitchen of our 100 year old house and I feel a draft by the bay window. Looks like it's time to get out the caulking gun and and go after those cracks where the cold air is infiltrating. This post by Steve Bertasso from his blog Decoding Green Building outlines how to eliminate air leakage. As a friend of mine who recently resealed the wall to floor joint in his enclosed porch said, "With every foot of sealant I put down I could feel the room get warmer."
Decoding Green Building: Green Your Home – Sealing Doors, Windows and other openings