Internet Explorer:
- Go to Tools (Alt+X in IE9) → "Manage add-ons"
- Select "Search Providers" on the left
- Check the "Prevent programs from suggesting changes to my default search provider" checkbox
- Click Close
Firefox:
Install the BrowserProtect add-on, which can detect and alert about changes to several configuration settings, including the default search engine.
Source: http://support.mozilla.org/en-US/questions/822455
Google Chrome:
Dev versions of Chrome, as well as Chromium, should have a feature called Protector, which you can activate by appending -Protector
to the Target field of your Chrome shortcut. After that, Chrome should warn you about any changes to your default search engine.
I'm not sure if this is available in the stable release, though.
Source: http://techdows.com/2011/12/google-chrome-prevents-defualt-search-engine-changes-by-malware.html