There can be viruses or key loggers or some malicious softwares in your computer which might be connecting to Internet in background secretly. There are also many softwares which use Internet without asking for your permission.
So the big question is:
How to check which application or software is using the Internet connection?
The answer is that there are more than one ways of doing it, lets see the methods one by one.
Using TCP-View Utility:
To see what applications are connected to the Internet or what applications are using Internet, you can use a small utility by Microsoft Sysinternals called TCPView. This is a small and handy utility which you can download here and run.
When you run the utility it will display all the applications which are connected to Internet. See the image below (Click to enlarge):
Connections which change state from one update to the next are highlighted in yellow; those that are deleted are shown in red, and new endpoints are shown in green.
And don’t worry about SVCHOST.EXE, this is a windows task which keeps running in background and will be displayed in this list several times.
This will give you an idea of what processes are using Internet connection. If you find anything suspicious, search that file and delete it if you think that its something that you have never installed something like that.
Also there is a geeky way of doing this without using the TCPView Utility,for that open the command prompt and type the following command and it will display all the applications connected to Internet, it will refresh the list every two seconds.
netstat -b 2
Have a safe and secure Internet Surfing!