Condensation is generally not a big problem in winter. Unless the office has an overly agressive humidifier, there is usually very little moisture in the air. Using the cold can of cola experiment, you may find that there is very little condensation on the outside of the can during winter months.
If you are really worried, then a heat pack and some insulation should do the trick. A very effective insulation can be made very cheaply by cutting a blue foam closed cell sleeping pad from walmart to make a close fitting box for the laptop. Use contact cement to glue the pieces together. Unless you have a very long ride, your laptop will probably arrive toasty warm.
Other ideas: A reusable heat pack (save money over disposables). One or two electric socks stuffed in next to the laptop. Run the laptop with a CPU intensive application until you are ready to leave so it will be at maximum temp when you put it in the bag.