Windows Defender is an excellent feature in Windows that scans your system for malware and spyware. Windows Defender helps your system to keep up in good health and warns you for any possibility of infection. However if you want to turn it on or off in Windows 8 here is how you can do it:
1. Go to Control Panel
2. Click Windows Defender
3. Go to Settings and check/unchek Turn on Windows Defender to enable or disable it.