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Go to start menu --> control panel --> programs and features or add/remove program --> here you can see if Microsoft Security Essentials is installed.
If it's not, we can download it as it's free. Below is the manual for that device. http://downloads.dell.com/Manuals/all-products/esuprt_desktop/esuprt_inspiron_desktop/inspiron-3847-desktop_Owner%27s%20Manual_en-us.pdf
Download security essentials here: http://windows.microsoft.com/en-us/windows/security-essentials-download
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