Seems that your system is affected with some kind of malware/trojan!
If you have antivirus/ antimalware installed, do a complete system scan!
or else, Download & install ,
1. Avira Anvivir - http://www.free-av.com/en/download/index.html
Avira has very good internet security, real time updates, great overall protection.
2. Malwarebytes - http://www.malwarebytes.org/
Malwarebytes is one of the best malware scanners.
Scan the system & Delete the threats found!