It sounds as though your computer has been hijakced by some spyware or malware.
You need to do a scan with some malware products to see if it finds one or more. I like malwarebytes, but there are others.
Download it, update it, then run a full scan.
http://www.malwarebytes.org/
There are other malware product and it is OK to install and run more than one.
Note this is NOT a virus so your anti virus software will probably not find it.