Windows Defender v Microsoft Security Essentials

It all depends on what edition of Windows you are using

Windows Vista

  • Windows Defender installed by default

  • Installing Microsoft Security Essentials does more and will DISABLE Windows Defender

Windows 7

  • Windows Defender installed by default
  • Installing Microsoft Security Essentials does more and will DISABLE Windows Defender

Windows 8

  • Windows Defender installed by default
  • You CANNOT install Microsoft Security Essentials !

http://www.microsoft.com/security/pc-security/spyware-remove.aspx