BMW moto-rad sells sets of tire chains for bikes, but they're 200Euro per set. If you're really that set on wanting to ride in the snow, check out the pics for the motorad ones and make up a set for yourself(pretty easy with basic tools) or check for the manufacturer of the old Ural tires. They used to be available with studded tires.
We must select the illusion which appeals to our temperament and embrace it with passion, if we want to be happy
--[B]I support the separation of church and hate[/B]--