You need to take a holistic approach - the fleas are probably in your home (carpets, bedding &c) and are only going on the dog to feed. They won't survive outside in this weather so they're having a lovely time in your nice, warm house! You can get a spray called Staykill to use on the soft furnishings (make sure you spray all around the edges of carpets, where they meet the wall, in particular) but don't use it on the dog and when you wash the bedding, put it on the hottest was you can.