How can i track my bandwith usage ?

I have registered to an internet service provider which gives me a fixed fee of say USD 10 per month for a max use of 250 Mb per month. Any extra byte used beyond that would be charged a more expensive fee. I use windows vista. Is there any available tool in Windows which I can use to track how many Kb i have used per internet session? If not, is there any freeware I can use & what's the name? Thank you.