Most likely, there is other software running that keeps restoring the extension. Try opening the Windows Control Panel, Uninstall a Program, and click the "Installed on" column heading to bring the most recent infections, er, additions, to the top. This also will usually reveal what was installed together that may have been part of a bundle. I would get rid of everything that you don't recognize and useful and valuable. And maybe even the free software it rode in on. Hopefully then removing the extension will solve it permanently. Note that some utility software may help prevent browser changes by undoing your changes. These features often go by names such as "surfing protection", "browsing protection", "home page protection", etc. If you have software with that kind of feature, make sure to turn it off while making settings changes to avoid having your own changes canceled/reverted.