Some 3rd party antivirus (AV) programs may turn off Windows Defender
when they are installed to be used instead, and will not allow Windows
Defender to be turned back on until the 3rd party AV has been completely
uninstalled.
To Turn Off Windows Defender in Windows 8
1. Open the Control Panel (icons view), and click/tap on the Windows Defender icon.
2. In Windows Defender, click/tap on the Settings tab, Administrators in left pane, uncheck the Turn on Windows Defender box, and on Save Changes. (see screenshot below)
3. If prompted by UAC, then click/tap on Yes.
4. You will now notice the message from Action Center notification area icon letting you know that Widows Defender has been turned off. (see screenshot below)
NOTE: If you click on this message balloon while open, it will turn on Windows Defender again.
5. Click/tap on Close. (see screenshot below)
OPTION TWO 
To Turn On Windows Defender in Windows 8
NOTE: This is the default setting.
1. Do step 2 or 3 below for what you would like to do.
2. Click/tap on the Action Center icon in the taskbar notification area, click/tap on either one of the Turn on spyware protection or Turn on virus protection links, and go to step 4 below. (see screenshot below)
A) Click tap on the Turn on now buttons for either one of the Virus protection or Spyware and unwanted software protection messages, close Action Center, and go to step 4 below. (see screenshot below)
Note
4. Windows Defender will now open with it's security status as green to let you know that both Windows Defender and real-time protection has been turned on. (see screenshot below)
To Turn Off Windows Defender in Windows 8
1. Open the Control Panel (icons view), and click/tap on the Windows Defender icon.
2. In Windows Defender, click/tap on the Settings tab, Administrators in left pane, uncheck the Turn on Windows Defender box, and on Save Changes. (see screenshot below)
3. If prompted by UAC, then click/tap on Yes.
4. You will now notice the message from Action Center notification area icon letting you know that Widows Defender has been turned off. (see screenshot below)
NOTE: If you click on this message balloon while open, it will turn on Windows Defender again.
5. Click/tap on Close. (see screenshot below)
OPTION TWO NOTE: This is the default setting.
1. Do step 2 or 3 below for what you would like to do.
2. Click/tap on the Action Center icon in the taskbar notification area, click/tap on either one of the Turn on spyware protection or Turn on virus protection links, and go to step 4 below. (see screenshot below)
A) Click tap on the Turn on now buttons for either one of the Virus protection or Spyware and unwanted software protection messages, close Action Center, and go to step 4 below. (see screenshot below)
Note
If you do not have the Virus protection or Spyware and unwanted software protection messages in Action Center like above, then:
A) Expand Security, and click/tap on both the Turn on messages about virus protection and Turn on messages about spyware and unwanted software protection links like below to see them.
A) Expand Security, and click/tap on both the Turn on messages about virus protection and Turn on messages about spyware and unwanted software protection links like below to see them.
4. Windows Defender will now open with it's security status as green to let you know that both Windows Defender and real-time protection has been turned on. (see screenshot below)
