Many riders opt to keep a second bike for the winter months. The sand and salt that gets put down on the roads can be harsh on components, so a more rugged rig is more suitable than a high-performance racer. In any case, you’ll want to mount wider, more durable tires on your wheels for better grip on slippery roads and to prevent punctures. Standing by the side of the road fixing a flat when it’s freezing is not fun and you should do what you can to avoid it.
Fenders will keep you much dryer in the wet, and are pretty-well obligatory if you are riding with others. You don’t want to be the jerk spraying dirty, cold water into the faces of the riders behind all day, and you don’t want anyone spraying cold, dirty water into your face either. After each winter ride, wash your bike right away. Hosing off the mud and salt and oiling your chain will make your machine last much longer.