Block of Adult site

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While I suft internet at home or office suddenly some adult site float in the screen but I or any user of my home PC and office PC cannot use it or never visit this type of site. It is very embarrassing and disgusting to everyone when surfing internet from home and suddenly and unexpectedly adult site float in screen. Do you have face such situation if so how you tacle it? Can anyone tell me how to blog unwanted adult site from their embarrassing tresspass to my PC ?