This is caused by malware which is used to form a botnet. There is usually no sign about this running on your computer, because it sits there waiting for it to be activated by its creator. When it's activated though, it will slow down your entire computer and cause a massive amount of network traffic. The best solution is preventing your computer from being infected in the first place by installing a anti-malware solution which is up-to-date with the latest malware information.
