There are many processes or applications that can connect to the Internet on their own and share the connection with you thereby decreasing your Internet speed, for example, the users who have enabled their Anti Virus to download their definition files automatically usually face less speed sometimes.
Apart from this you can also know if there is any malicious program in your computer that is sharing your Internet Bandwidth while running in the background.
This is done by using an application called TCPView which is provided for free by Microsoft Sysinternals. The advantage of using this small tool is that it not only classifies the processes or programs that are using the Internet but also highlights those which are changing state time to time.
The same thing can be done by typing netstat -b 4 (4 signify the auto update time in seconds) in the command prompt but I like TCPView more because:
• It makes it easier to figure out the name of the process.
• Highlights the status changing processes with different colors.
• You can kill the suspicious process by right clicking in the name in the software itself.
• No need to install.