Your body receives much higher doses of Vitamin D from sunlight than from supplementation. For instance, your body produces about 10,000 units of Vitamin D in response to about a half hour of sun. Most Vitamin D supplements should not be taken over a couple thousand units, at least that's about the best consensus you'll find in the literature.
normal, if you're having a little bit of trouble in the winter, they make special desk lamps that you can buy that may help with your moods. Do a google search for "seasonal affective disorder" and "light boxes" and that should get you going in the right direction. I've heard good things.