If you really want to keep wearing something similar to the five fingers over the winter take a look at some wetsuit boots. They come up your calf a bit, have a flat decent sole on them and will keep your feet quite warm. Keep in mind that your feel will probably sweat a bit in them, but that thin layer of water is what will end up keeping them warm.
Example: http://www.billabong.com/eu/product-cat/225/booties#_