Windows Defender is Microsoft Security Essentials, and with Windows 8 you get the added benefit of SmartScreen protecting whole your OS.
If you wanted to replace Windows Defender, make sure it's Compatible with Windows 8.
Malwarebytes is a good standalone on-demand scanner.
HitmanPro might be addition, but zero benefits unless you're infected, but I'm ~sure MBAM can remove those for free.
Keep copies of your personal files backed up and a recovery disc of Windows 8 installation for emergency.