Assuming you are using IE, look in your list of "Addons" and see if there is any item there you don't recognize.
Install Super Antispyware. Run it in safe mode. Allow it to quarantine whatever it finds.
http://www.superantispyware.com/Run the online scan for Bit Defender in normal mode. Allow it to quarantine whatever it finds.
http://www.bitdefender.com/scan8/ie.htmlPlease let us know the results of the scans and whether your problem is solved or not.