I know there must be a lot similar questions on SU. Let me explain my setup first.

1. I have 4-5 PC, Laptops and Few Android Phones in my office.
2. To get them on a network , I have a UTStarCom, WA3002G1 ADSL2+ router with a landline broadband connection which has nothing to do with any PC except the configuration settings.
3. Broadband channel is always on, we need to switch on the router and the internet is ready for us.
4. No Internet Connection sharing is done via any PC.
5. I have a limited 20GB monthly plan, which is consumed in 10-20 days, depending upon the download requirements.

So in the above case, i need some suggestions from you:

1. How do I monitor my Internet Bandwidth along-with the connected systems, realtime? Any free opensource tool available?
2. Tweaks / Changes in PC to save bandwidth as my ISP do not have any Unlimited plan.
3. PC and Laptops are Windows XP and/Or windows 7. Either of the platform tools are welcome.