Buy one of these kits: Invalid Link Removed
Comes with double sided tape, put the tape around the window frame, cut a sheet of plastic to the size of the window + frame, attach plastic to tape, covering the window. Use hair dryer to shrink the plastic, making it wrinkle-free and making a tight seal. The air space between the window and plastic becomes your insulating layer, no more drafts. Works pretty damn well.
Provided there aren't huge gaps/leaks elsewhere. I spent two days sealing the window frames with silicone last year before I put up the plastic.