I open my browser and without me doing anything my search engine gets changed and sometimes my default page changes. How can I stop them from hijacking my search engine.
The solution sadly is not to stop it from happening but how to fix it afterwards. The cause was you visiting a web site that probably offered some free download of something or making it easier to do something. In the really fine print was something saying to help your finding things it will change your search engine. But of course you never read the fine print.
So to fix things it will depend on the browser you use. Chrome, IE, Firefox Safari. So manually go to your favorite search engine, and search for Browser default URL, Browser default search engine. This will tell you how to change things back.
You also should search for Browser Add-in or Extension removal. Add-ins extensions are things added to your browser to make your visit to some web site experience better. But often it causes problems in performance. Remember to be very careful to not allow things to modify your system without knowing you really want/need.
Another reason to image backup your system. Viruses aren't the only things that can cause serious problems. Backup, backup, backup!!