All depends on how much you want to invest in winter bike clothes. You can ride down to 32*F or so with tights, a baselayer, long sleeve jersey, and light wind jacket. You'll also want gloves and shoe covers. If money isn't an issue, you can walk into most LBS and get set up for $200-300.
If you don't want to spend a lot, you can get by with nylon "wind pants" or even sweatpants on your legs, and an undershirt/sweatshirt on top. For hands, just put on some knit gloves over your shorty cycling gloves.
Don't forget the feet. Most cycling shoes are ventilated for summer, and cold feet can make a ride misterable. I think shoe covers or booties are worth the money in comfort, but you can improvise there too by using a pair of heavy wool socks over your cycling shoes (slit or cutout the bottom for your cleats). Some guys throw plastic baggies over their shoes just to block the wind, but they never seemed to do much for me.