In reply to Paul Manson:
If you are getting cold feet, first make sure your feet can move about inside your boot. Close fitting boot and no foot movement means lower insulation and less blood flow, means risk of cold injury.
Its worth getting boots a little too big so you can keep your feet warm.
Also, thick socks help. Again, make sure you have enough room to keep your feet moving around.
In terms of pure cold, I have used my normal trainers and socks in 25 below freezing in Canada (not in soft snow!) and my feet have been fine. The rest of my body was well wrapped up, but normal trainers provided enough insulation. Thick sole, a bit of padding, and enough room to move my feet.