Free Anti-virus software

For windows 8 user:

Windows 8 comes with pre-installed  Windows Defender.
In order to activate it:
Step 1: Press windows key and X key , a menu will pop up on the right bottom corner.

Step 2: Click on Control Panel ; control panel window will pop up

Step 3: Click on Windows Defender: In normal condition the defender should be active and the color should be green. If this is the case your computer is under protection.

If it is in red color: this means your computer is not under protection. All you need to do is click on Turn On (button in red).

while you are on windows defender window you can click on Update and let it to download latest version of windows defender automatically.

It is simple as you see in steps above.

For Windows 7 users:
Option 1: Windows Defender

There are two options for win7 users; win7 comes with pre-installed windows defender too.
If you want to use windows defender go to Start --> Control Panel --> Windows Defender

Windows defender on win7 looks like as seen on pic below:

If you are using it for the first time probably you will need to update it. Click on Check for updates now button and let it install the latest version of Windows Defender.

Once update completed it will look like as seen below

Option 2:

Microsoft Security Essential: to download Microsoft Security Essential please click http://www.microsoft.com/en-ca/download/details.aspx?id=5201 link this will take you to official Microsoft download site.

After you installed the Microsoft Security Essential on your window 7,  it will automatically checks for updates. I will recommend you to install Microsoft Security Essential for a better protection.