It is probably the result of a piece of malware rather than a virus, though that is also a possibility.
Download and install Lavasoft AdAware (the free edition) from
here, then download and install Malwarebytes Anti Malware (the trial version) from
here. Then reboot your PC.
Start AdAware and make sure you get the latest updates, then run a scan. When the scan finishes, get it to delete everything it has found.
Start Malwarebytes and get the latest updates, then run a scan and get it to delete everything it finds (you may need to restart your computer as part of that process).
The scans may take a while to run, but hopefully whatever is causing the problem will be removed.