You have a simple trojan is all, I suggest you first scan your computer with your anitvirus software to clear it of all possible virus infections and try once more, if the problem still remains you will need a specific Malware remover.
One I have used for many years now with great effect is Spybot Search and Destroy, available free here.
http://www.spybot.info/en/download/index.html.
Works well out of the box, update before scanning to make sure you have the latest detection rules.
Spybot has a number of other useful features that help protect you from this sort of problem. Well worth ten minutes downloading and setting it up.