batteries hate the cold. If your battery is over 5 years old just change it, don't gamble on it. If the battery is between 3 and 4 years old I suggest having a capacity check performed on it to ensure it will be able to withstand the rigors of the cold.
A battery that is old and tired may be easily able to start your engine when its 70 and sunny, but when its 15 and windy the only thing the battery will be able to do is make the starter go click, click, click, click.
Snow tires are a must especially if you are unfamiliar with driving on snow covered roads. When trying to get moving from a stop I've often found that using 2nd gear is easier becasue the wheels tend to spin much more easily when trying to start in 1st gear.
As 98SE suggested, if you don't have experience driving your car on snowy roads practicing in an empty parking lot is a must so you can learn how much more distance your car needs to come to a stop, how fast you can go around a corner before you start sliding and continue going straight, and if you try to hit the brakes with the wheel turned while sliding will usually put you into a spin. Go nuts in the lot trying to make your car loose control and then try to regain it. Keep trying until you feel comfortable or you get kicked out of the lot.
