Microsoft Security Essentials is the best and freeware antivirus program that protects your computer against root kits, computer viruses, Trojan horses, and spyware. Developed by Microsoft corporation. It uses the same anti-malware foundation as other Microsoft antivirus products, although it lacks the centralized management features of some of them. Microsoft Security Essentials works with Windows Vista, Windows 7 and Windows XP. In Windows 8 it is inbuilt, just its name is changed to windows Defender.
To install first download Security essentials (the link is giving below). You must have a genuine windows operating system on your computer; or else it won’t work. The setup package is approx 12 mb. then later, It updates anti-virus database. It takes approx 15 mins to 1hr; Depending upon your internet speed.
Windows automatically installs the all new important Updates as the software updates are available. First when have downloaded the software you need to update it as it is the initial setup process. Its better to have a Broadband connection or over 3g network. After finishing getting updates reboot your computer. Now you got the best protection for your PC. Windows Defender runs this in the background and notifies you when you need to take specific action. You can use it anytime to scan for malware if your computer is not working properly or you clicked a suspicious pages then it takes the action.
CPU : clock speed of 500 MHz or higher
Memory: 256 MB RAM or higher
VGA (Display): 800 x 600 or higher
Size: 9-12 MB installer.
OS: Windows XP / Vista / Windows 7. [32bit / 64bit]
Note: Windows Update won’t add any applications to your computer without asking for your permission.