what is using my internet bandwidth
In developing countries internet connection, even a 512 KBps connection is a luxury. People has to pay a premium for the bandwidth. Most of them are not using unlimited plans. Even the unlimited plans are having fair usage policies, which cap the transfer rate to 256 KBps once you reach a predefined bandwidth limit.
Think about some stupid programs leeching the bandwidth. Which uses up most of the bandwidth and makes the web surfing a pain. The better option is to use a firewall and configure it. I recommend Comodo firewall, the older version.
In windows 7 you have a good small utility which clearly shows all the internet connections with IP and ports. Name of the application is resource monitor, which can be accessed through performance monitor.
You may directly access the resource monitor by typing perfmon.exe /res in run box or MS-dos prompt. You can make a shortcut on desktop for easy access.
Once you identify the culprits use a firewall to block the unwanted applications from accessing internet.