Is it possible to let me know the browser you are using? like Internet Explorer or Mozilla Firefox or Google Chrome....etc. It's hard to assume your problem and the root of the cause since this is related to many issues. It's also important to let me know the Search Engine you are accessing e.g. Google or Bing etc....
This is often related spyware, but let's try from the Settings (Internet Setting of your browser).
When Low level Security is implemented on the browser's Settings, the pop-ups, tool-bars and Add-ons are freely accessing without any restriction which result in the diversion of search or sites on the Browsers.
Next, try to check your cookies and caches (you need to deleted them or clear them from the browser settings or using a specific application). When you open up the Browser settings (usually Internet Options/Options) you'll be able to see how to clear and how to delete the history, cache, cookies etc.. as shown below;
But this is manual and not everything will clean since the cleaning process will run while the browser is still opened. So for better results, I would suggest you to use a cleaner like Ccleaner.. this time I don't include the freeware because, it's dangerous and without proper instructions, you may loose files or downloaded softwares etc.. If you need it, i'll let you know with full details. The best thing is to run the cleaner after closing all the opened browsers and restarting the system right-away.
Please let me know, with a little details so that I can assist you better. I'll definitely help you fix this.
Good luck.
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FixYa.