Warning: Backup your browser bookmarks using a good app before you do this! All bookmarks will be lost from the Internet browser, including those that came pre-installed on your phone.
1. Remove any suspicious "Search" shortcut on your home screen panels.
2. Open Settings > Applications > Manage applications > All (tab on top).
3. Look for "Internet" in the list and open it.
4. Tap "Clear cache" (if enabled) and "Clear data".
5. Tap OK to any prompts that appear.
That's it, now you can exit out of Settings and open Internet. Check the settings if the default home page now points to either Google or your phone manufacturer. Set the various internet settings according to your preferences and restore the bookmarks you backed up.