Windows Defender, which has been part of XP, Vista, and Windows 7, has been tweaked in Windows 8 to include almost all features of Microsoft’s popular Security Essentials software. In other words, Security Essentials has been running in Windows 8 as Windows Defender (Read more 10 ways to work more securely).

This article describe how to us Windows defender in Windows 8:

Step 1: Switch to the Start screen, start typing Windows Defender and then press Enter key to open the same

Step 2: Under Home tab, select Quick or Full scan option and then click Scan now button to begin scanning your PC

Step 3: If you want to scan a specific folder or drive, select Custom scan, click Scan now button and then browse to the folder or drive that you want to scan

Please note that with default settings Windows Defender doesn’t scan your removable drives, such as USB flash drive, while running a full scan. To include a USB drive in the full scan, switch to Settings tab, click Advanced and then enable Scan removable drives option.

Via intowindows.com