It sounds like you may have either a virus, or some spyware or adware. Run your antivirus and also some anti-spyware and anti-malware programs
For antispyware/adware/malware I use Superantispyware and Malwarebytes and both are good programs. I would run them both. These are in addition to your antivirus program. They are free to download and use and can be found here :
Superantispyware
http://www.superantispyware.com/ click on the free download button
Malwarebytes
http://www.malwarebytes.org/ click on the download free version button