Google a program called NetMeter.  Its free, small, runs in the system tray, and keeps track of all kinds of web use stats for you.  It will tell you if you are reaching your quota, and also predicts what your download total for that period is going to be based on activity to date.

One other neat feature is that you can run a transparent graph that shows dl speed and activity.  I run this on my home computer and once you get used to how it should look under normal browsing, its very easy to pickup when there is internet activity going on that you might not want.  Eg one of those file share programs is loading on startup and uploading data without you knowing etc.