How to return computer to original condition

[RickFlagler]

computer 10 years old running MS 10. Something got into computer, and I cannot return it to its original state. Backgrounds have changed colors, pages are black with white type, page formats have all changed. When I looked at Add or Delete Programs, most if not all operating programs have been replaced in one day. I did security check which ran for hours and no results. I have new programs installed for PC health and Driver updates which I never requested. Something has taken over this computer. Help

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