I have windows 7 and to monitor and control my bandwidth/broadband/whatever I used Netlimiter. There is a 30 days free trial but some of the functionality kept working even after the trial expired for some reason.
It let you limit the maximum transfer speed to whatever you want. You can also see what software is the most data heavy (download and upload).
I don't remember if it lets you stop a program from downloading anything.

I find my 120GB small but workable so I can't imagine 20Gb per month.

About Chrome, I agree with Pestilence. Ever since I stopped using IE (a few years ago) I have been using Firefox. While it's good, Chrome usually performs better. The gap is not large enough to justify installing it.
The reason I installed Chrome in the first place is because some sites didn't work properly in Firefox. Yesterday again, I had to use Chrome to access a website because Firefox only displayed a message error.