Microsoft's own free anti-virus software - Microsoft Security Essentials - is easily understandable and does not require much configuration. This is a replacement for Windows Defender in Windows Vista and 7, adding anti-virus and network protection functionality. Scanning network traffic is available for Windows Vista and 7 only, though - the required Windows Filtering Platform is not available in Windows XP.
Microsoft Security Essentials is available for Windows XP, Windows Vista and Windows 7, both for 32-bit and 64-bit editions. The latest version, 4, was released in April, 2012.
Windows 8 has a special, much improved version of Windows Defender that provides full real-time protection just like Microsoft Security Essentials. Therefore you cannot install Security Essentials in Windows 8.
Because Microsoft Security Essentials replaces the traditional Windows Defender in Vista and 7, the latter is not allowed to start anymore. In case you see error messages about Windows Defender at startup, or you see failed updates in Windows Update, move here to resolve this issue.
If you have some other anti-virus program installed, remove it first!
Because running more than one anti-virus program at the same time makes your computer slow, it is very important to first uninstall the currently present one. If you do not have an anti-virus program installed, move down here.
Click to show or hide uninstall instructions for Windows XPIn Windows XP, click the Start button or press Windows Key on your keyboard and choose Control Panel.
Click Add or Remove Programs. First picture is for users of Control Panel in Category View, the second one for users of Control Panel in Classic View.
Find your current anti-virus program, click on its name and then click Change/Remove or Remove button (button name depends on the program). Then follow the uninstall wizard to remove the program. You probably have to restart your computer after this. You will also see Security Center's red shield icon in Taskbar Notification area, this is normal because you have temporarily no antivirus program installed.
Now move down to downloading part.
In Windows Vista and 7, click Start button or press Windows Key on your keyboard to open Start menu. Then type "remove" into Start menu Search box and click Add or remove programs.
Find the anti-virus program you want to remove, right-click on it and then click Uninstall or Uninstall/Change (depending on the options the program provides). Then follow the uninstall wizard to remove the program. You probably have to restart your computer after this. You will also see Action Center flag icon with red cross in Taskbar Notification area, this is normal because you have temporarily no antivirus program installed.
Downloading and installing Microsoft Security Essentials
Open Microsoft Security Essentials home page and click the Free* download button.
If Windows version is not detected, you'll have the Select your version combo box. Choose the correct version of Windows (32- or 64-bit) and then click the Free* download button.
After downloading and launching the setup file, Microsoft Security Essentials Installation Wizard starts, click Next.
License Terms page opens, click I accept.
If you are very-very concerned about your privacy, do not join Customer Experience Improvement Program. Choose I do not want to join the program at this time and then click Next.
Next, setup offers to optimize security by turning on Windows Firewall in case it is turned off and no other firewall program is installed and active. In most cases, you can leave the If no firewall is turned on, turn on Windows Firewall (recommended) check box selected and click Next.
Now setup is ready to install Microsoft Security Essentials. Click Install.
The installation part takes a little while. After it is complete, leave the Scan my computer for potential threats after getting the latest updates box checked and click Finish. This will open Microsoft Security Essentials, start an update process and then run a quick scan (around 10 minutes long).
That's it for the installation part. 
Upgrading Microsoft Security Essentials
If Microsoft Security Essentials' icon remains yellow-orange for long time, it could mean that there is an upgrade to the program available. Click the icon and then click Open.
In the main window there should be a message "A new version of Microsoft Security Essentials is available. You should upgrade now". Click the big Upgrade now button.
Windows Vista and 7 users will see User Account Control popping up. Click Continue or Yes to continue with upgrade:
Microsoft Security Essentials Upgrade Installation Wizard opens. Click Upgrade.
In the License Terms page, click I agree.
Again, if you are very concerned about your privacy, click I do not want to join the program at this time to reject the Customer Experience Improvement Program. Then click Next.
The upgrading might take several minutes to complete. After the new version has been installed, you will see the following screen. Click Finish.
And this completes the upgrade. 
Possible Windows Defender error messages after installing Microsoft Security Essentials
In some cases, Windows Defender displays an error message after starting or restarting your computer. The "Windows Defender is turned off" message can be safely ignored by clicking Close. Remember, Microsoft Security Essentials actually replaces Windows Defender in Windows Vista and 7 and therefore the older service is not allowed to start anymore.
If you try to start the Windows Defender service manually, you'll see the "Windows Defender service on Local Computer started and then stopped" dialog. Click OK. You might also see some failed updates in Windows Update in case Windows Defender is still enabled.
The dialog on computer startup appears because Windows Defender service is set to start automatically. To stop the message from appearing again, follow the steps below:
- Open Start menu and type "services" into Search box. Click Services.
- Find and right-click Windows Defender service. Select Properties from the menu.
- Set Startup Type to Manual. Click OK to close the service properties window.
