It will be mostly because of some unwanted browser plugins or some advertisement applications installed on yo your machine (like tool bar applications).

What I would suggest is. go to you application wizard(add or remove application) see for such unwanted application installed on your machine and remove them and restart your browser.

hope this works.

It will probably be because of some unwanted browser plugins/extensions or some adware (software designed for the purposes of advertisement) installed on your machine (like toolbar applications).

What I would suggest is, go to 'Uninstall a program' and look for such unwanted applications installed on your machine and remove them, then restart your browser.