I agree with william v. However, if you do have trim on the inside then carefully remove it and see if there is gap between window and rough frame. Use fiberglass insulation pieces to fill the gap, do not pack it in tightly as this reduces the r value. If the gap is very small use a good caulk to fill the void. I would avoid using the foam stuff, as it can expand dramatically and cause window frame to warp. Put the trim back on and your draft should be taken care of. When the weather gets nice, carefully caulk the outside of window frames. Realize the above assumes the drafts are coming from around the window frame. If the windows are not made right, the draft could be coming from other areas of the window.