Microsoft offers Microsoft Security Essentials free antivirus for Windows XP/ Vista/7 computers, not for Windows 8 because its not compatible but you can install it anyhow. MSE offers real-time protection against viruses, spyware and Trojans. Users using genuine windows can install MSE at free cost, Microsoft provides definitions for MSE regularly to detect new threats.
Microsoft Security Essentials Standalone/Offline installer
Generally to download MSE from Microsoft website, you need to visit its download page and select version of OS you’re using and click on “Download now” button. Without this hassle you can download MSE with a single click a from its official page and here is how.
Visit this link in browser and download 32-bit or 64-bit version of MSE to your computer.