Windows Defender 

• Error message when you try to update Windows Defender on a Windows Vista-based computer: "Error found: Code 0x80240029"
  http://support.microsoft.com/kb/934562

• The installation path may be shorter than the path that you specify when you install Windows Defender
  http://support.microsoft.com/kb/925557

• How to manually download the latest definition updates for Windows Defender
  http://support.microsoft.com/kb/923159

• Error message when you run Windows Defender: "Error Code 0x800106ba"
  http://support.microsoft.com/kb/931849

• Error message when you try to start Windows Defender on a computer that you recently upgraded to Windows Vista: "Application failed to initialize"
  http://support.microsoft.com/kb/935511

• How to troubleshoot definition update issues for Windows Defender
  http://support.microsoft.com/kb/918355
  "common error codes are 0x8024402f or 0x80240016"
  "Other errors: 0x8024402c, 0x80240022, 0x80004002, 0x80070422, 0x80072efd, 0x80070005, 0x80072f78, 0x80072ee2, 0x8007001B"

• Vista - Windows Defender

  • Turn Windows Defender on or off
    http://windowshelp.microsoft.com/Windows/en-US/Help/31d797aa-091d-4d67-a556-dbfaf21bf0dc1033.mspx 

  • Using Windows Defender
    http://windowshelp.microsoft.com/Windows/en-US/Help/4ef8da25-133e-4947-99c1-5c7951d48a151033.mspx 

  • Understanding Windows Defender real-time protection
    http://windowshelp.microsoft.com/Windows/en-US/Help/8b8917f4-88c7-4470-88ae-0a06987c20fd1033.mspx 

Microsoft Malware Protection Center
Threat Research and Response
http://www.microsoft.com/security/portal/

Windows 8 -

Hosts file is detected as malware in Windows Defender:
http://support.microsoft.com/kb/2764944

Why does the prompt "Windows defender has been turned off" appear when Windows 8 load Windows Defender? (Made Easy Series)
http://support.microsoft.com/kb/2820875