For a personal project, I needed a program that would track daily bandwidth utilization on a few Windows laptops I use. So far, Bandwidth Monitor seems to be the best choice with one caveat — it does cost $19.95 to register after the 30 day trial period.
There are freeware bandwidth monitors for Windows available and I tried a few of them, but they tended to be wildly inaccurate in their reporting of bandwidth usage.