"Windows Defender" is a "Microsoft" application that helps protect your computer from spyware, viruses and other malicious software. Follow the steps described in this article to enable "Windows Defender" and keep your computer safe from malicious third parties.
Step
Method 1 of 3: On "Windows 8"

Step 1. Click “Start” on the “Windows 8” screen and type “Windows Defender” in the search field
Click the “Windows Defender” icon once it appears in the search results. This app will be launched after that.

Step 2. Click the “Settings” tab, and click “Real-time protection” on the left panel

Step 3. Check the box next to “Turn on real-time protection (recommended)”, and click the “Save changes” button located at the bottom of the window
Now "Windows Defender" is active.
Method 2 of 3: On "Windows 7"

Step 1. Click the “Start” button on your “Windows 7” desktop

Step 2. Type “Defender” in the search field and select “Windows Defender”

Step 3. Click “Tools” in “Windows Defender” and select “Options”

Step 4. Click “Administrator”

Step 5. Check the box next to “Use this program”

Step 6. Click “Save”
Now "Windows Defender" is active.
In some cases, you may be prompted to type your administrator password before "Windows Defender" saves your settings
Method 3 of 3: On "Windows Vista"

Step 1. Click the “Start” button and select “All programs”

Step 2. Click “Windows Defender”
This will launch the application.

Step 3. Click “Tools”, then select “Options”

Step 4. Check the box next to “Use Windows Defender” under the section named “Administrator options”

Step 5. Click “Save”
Now "Windows Defender" is enabled on your "Windows Vista" system.