Now you can bike in cool and cold weather in clothing not made for biking, jackets, heavy sweatpants etc but trust me, none of these will keep you warm on a bike ride. And being cold on a cold day makes you HATE biking. It takes time to assemble winter gear, but if you do it slowly, in time you have a great winter bike outfit. You only need ONE. Go home and wash it and have it ready for the next ride.
First thing to think about is your head, you need some different stuff in the cool weather. I highly recommend a good head cover, something similar to a bathing cap only made out of the nylon stretchy material. Head bands for extra warmth on the ears are also a must. In really cold weather, a good balaclava is a nice thing to own as well.
Next thing to think about are your hands. Now I was in Sports Authority the other day and they sold gloves made by UnderArmour. I am going back and buying a pair. They are thin nylon and are made to be worn under your bike gloves. Now in cool weather your summer bike gloves and the Under Armour gloves would be fine. In cold weather, you will need winter biking gloves.
Next thing is your feet. Good thick winter bike socks are invaluable and good shoe covers. These two items can be found in most bike stores. I like the shoe covers made by Terry
Now for your body. Leg warmers and arm warmers are super to own. I have a pair of Gore Windstopper Arm warmers that are very good. Found mine at the end of the season at Performance Bike Store in Paoli. Got my leg warmers there as well.
One pair of long tights. I only buy pants from Terry Cycling but there are many many companies selling cycling bottoms.
One very good long sleeve undergarment. Patagonia makes some superb base layers. Buy ONE good piece and it will last many seasons.
Last but not least, a good jacket. I have ONE jacket, a Gore Windstopper. I shop the Net and find them on sale. Get bright yellow if you can. Cyclists are hard to see in the winter with the long shadows and lights. If you search places like Nashbar or Sierra Trading company, you can often find a Gore Windstopper at a great price. I just found one for my son for $79 which is a great price.