Limit accessing my computers like installing programs and software

Dear administrator, well I am a businessman who owns a store that specializes in IT goods especially computers. My computers operate normally just like any other that is sold anywhere and those owned in homes. However, they develop complications that hamper their speed since my clients keep on introducing new programs and software in the systems after purchasing them from me. I have plans to activate guest accounts so as to have a limit to what they can add on to the computers and I was just wondering whether this account would get avenue to administrator programs, games and other software. I am a visitor to your site so please help me.
