Got a wired keyboard or wireless? How many search fields / add-on toolbars do you have at the top of the web browser window? Does the problem occur in every web browser? (For example, if you're using Internet Exploder, try Firefox or Google Chrome.) If improvement in different browser or if you have more than one search box up near the address field of your browser window, go through your installed applications (Start menu --> Control Panel --> Programs and Features) and uninstall anything that sounds toolbar-y or spammy, or any security software you didn't intentionally install.
If none of that helps, consider downloading and installing
Malwarebytes and let it scan and remove any threats it finds. During install, be sure to opt out of the free trial of the full version unless you're interested in buying the full version for its added features.
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