Advice from Google on what to do if Chrome's settings have been changed by an unwanted program or extension, or if your default search engine has been changed : from a thread in the Google Product Forums
2. Select Settings.
3. Click Show advanced settings and find the "Reset browser settings” section.
4. Click Reset browser settings.
The post also has useful links to how-to-remove guides for a number of Potentially Unwanted Programs that are known to alter browser settings. Questions are often asked here about Babylon and Conduit, so these guides should be useful (for Chrome users, at least).