According to the National Institute of Health's Office of Dietary Supplements "Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60%. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D. Sunscreens with a sun protection factor of 8 or more appear to block vitamin D-producing UV rays." And according to the Medical College of Wisconsin if you live in Boston, MA, from November through February you will not get sufficient exposure to synthesize vitamin D. This would, logically, include anyone living further north than Boston, MA.
One suggestion to increase vitamin D is to go out for a short period of time, in the spring and summer months, without sunscreen and then apply after getting some exposure. Since how much exposure you need is determined by a number of factors, skin color, latitude, climate condition, and season, it is not easy to know how much is enough but 10-15 minutes a few times a week is generally considered reasonable.
edit: Apparently now there is some controversy about SPF numbers as evidenced by this NYT article. The article gives several important pieces of advice including to use the recommended amount of sunscreen; if you don't you're not getting the coverage that you think you are. Another important point is that you are looking for appropriate UVA and UVB coverage. Also mentioned is the fact that even SPF100 does not provide 100% coverage.
provide photo courtesy of freedigitalphotos.net