A search engine hijack may not necessarily be a problem on your computer so the answers in that other question linked in a comment may not help in the format they are specified.
What I mean is that a search engine hijack can be caused by changes to your network router instead of your computer.
See for example this article about a backdoor in a particular Arris router; the author of this article says (in a comment to his article) that criminals from certain country use this type of vulnerabilities to change users DNS (with malicious intent).
To determine whether this is the case you can:
- Check if any other browser is similarly hijacked (if it's caused by the router, all browsers would likely behave in the same way).
- User a free service such as the F-Secure Router checker or e.g. the trial version of F-Secure Freedome which also protects against router hijack.