Windows defender gives error message on windows startup

I believe it means the Windows Defender service is disabled, but you haven't also disabled it through the "Windows Defender" applet in the Control Panel. There's a small program that is set to run on start up that works together with the Windows Defender service (probably to provide the tray icon) which isn't disabled/enabled with the service.

Either go into the Control Panel and uncheck the box "Use this program", or get a utility such as Sysinternal's Autoruns and stop Defender from launching on startup. This is assuming you don't want to run it.

If you did want to run it, you probably just need to go to Administrative Tools -> Services and make sure Windows Defender is set to "Automatic" and is enabled.

From : fix-things-windows-defender-corrupt

The problem lies in corrupted registry settings for the program. If you have a similar problem to mine (Defender in Vista seemingly installed, but getting a "can't initialize" error), try the following (originally posted here by cdninja)

Step 1: Back up Registry ==================

1. Click Start, type "regedit" (without quotation marks) in the search bar and press Enter.

2. In the Registry Editor, click File menu, and click Export.

3. In the Save In list, select the folder where you want to save the backup. 4. In the File Name box, and type a name for your backup file, such as "Options" or "Backup".

4. Click Save.

Step 2: Create and Run the Registry File ==================

1. Click Start, type "notepad C:\register.reg" (without quotation marks) in the Search bar, and then click OK. Choose Yes when you are prompted to create a new file.

2. Copy the following texts between the star marks and then paste them into the opened Notepad window:

************************BEGIN**************************

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend]
"DisplayName"="@%ProgramFiles%\\Windows Defender\\MsMpRes.dll,-103"
"ErrorControl"=dword:00000001
"Group"="COM Infrastructure"
"ImagePath"=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,\
74,00,25,00,5c,00,53,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,73,\
00,76,00,63,00,68,00,6f,00,73,00,74,00,2e,00,65,00,78,00,65,00,20,00,2d,00,\
6b,00,20,00,73,00,65,00,63,00,73,00,76,00,63,00,73,00,00,00
"Start"=dword:00000002
"Type"=dword:00000020
"Description"="@%ProgramFiles%\\Windows Defender\\MsMpRes.dll,-3068"
"DependOnService"=hex(7):52,00,70,00,63,00,53,00,73,00,00,00,00,00
"ObjectName"="LocalSystem"
"ServiceSidType"=dword:00000001
"RequiredPrivileges"=hex(7):53,00,65,00,49,00,6d,00,70,00,65,00,72,00,73,00,6f,\
00,6e,00,61,00,74,00,65,00,50,00,72,00,69,00,76,00,69,00,6c,00,65,00,67,00,\
65,00,00,00,53,00,65,00,42,00,61,00,63,00,6b,00,75,00,70,00,50,00,72,00,69,\
00,76,00,69,00,6c,00,65,00,67,00,65,00,00,00,53,00,65,00,52,00,65,00,73,00,\
74,00,6f,00,72,00,65,00,50,00,72,00,69,00,76,00,69,00,6c,00,65,00,67,00,65,\
00,00,00,53,00,65,00,44,00,65,00,62,00,75,00,67,00,50,00,72,00,69,00,76,00,\
69,00,6c,00,65,00,67,00,65,00,00,00,53,00,65,00,43,00,68,00,61,00,6e,00,67,\
00,65,00,4e,00,6f,00,74,00,69,00,66,00,79,00,50,00,72,00,69,00,76,00,69,00,\
6c,00,65,00,67,00,65,00,00,00,53,00,65,00,53,00,65,00,63,00,75,00,72,00,69,\
00,74,00,79,00,50,00,72,00,69,00,76,00,69,00,6c,00,65,00,67,00,65,00,00,00,\
00,00
"FailureActions"=hex:80,51,01,00,00,00,00,00,00,00,00,00,03,00,00,00,14,00,00,\
00,01,00,00,00,60,ea,00,00,01,00,00,00,60,ea,00,00,00,00,00,00,00,00,00,00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend\Parameters]
"ServiceDllUnloadOnStop"=dword:00000001
"ServiceDll"=hex(2):25,00,50,00,72,00,6f,00,67,00,72,00,61,00,6d,00,46,00,69,\
00,6c,00,65,00,73,00,25,00,5c,00,57,00,69,00,6e,00,64,00,6f,00,77,00,73,00,\
20,00,44,00,65,00,66,00,65,00,6e,00,64,00,65,00,72,00,5c,00,6d,00,70,00,73,\
00,76,00,63,00,2e,00,64,00,6c,00,6c,00,00,00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend\Security]
"Security"=hex:01,00,14,80,04,01,00,00,10,01,00,00,14,00,00,00,30,00,00,00,02,\
00,1c,00,01,00,00,00,02,80,14,00,ff,01,0f,00,01,01,00,00,00,00,00,01,00,00,\
00,00,02,00,d4,00,07,00,00,00,00,00,28,00,ff,01,0f,00,01,06,00,00,00,00,00,\
05,50,00,00,00,b5,89,fb,38,19,84,c2,cb,5c,6c,23,6d,57,00,77,6e,c0,02,64,87,\
00,0b,28,00,00,00,00,10,01,06,00,00,00,00,00,05,50,00,00,00,b5,89,fb,38,19,\
84,c2,cb,5c,6c,23,6d,57,00,77,6e,c0,02,64,87,00,00,14,00,fd,01,02,00,01,01,\
00,00,00,00,00,05,12,00,00,00,00,00,18,00,ff,01,0f,00,01,02,00,00,00,00,00,\
05,20,00,00,00,20,02,00,00,00,00,14,00,9d,01,02,00,01,01,00,00,00,00,00,05,\
04,00,00,00,00,00,14,00,8d,01,02,00,01,01,00,00,00,00,00,05,06,00,00,00,00,\
00,28,00,15,00,00,00,01,06,00,00,00,00,00,05,50,00,00,00,49,59,9d,77,91,56,\
e5,55,dc,f4,e2,0e,a7,8b,eb,ca,7b,42,13,56,01,01,00,00,00,00,00,05,12,00,00,\
00,01,01,00,00,00,00,00,05,12,00,00,00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\WinDefend\Enum]
"0"="Root\\LEGACY_WINDEFEND\\0000"
"Count"=dword:00000001
"NextInstance"=dword:00000001

*************************END***************************

3. After you paste the above commands, please close the Notepad window. Choose Yes when you are prompted to save the file.

4. Save the file as a REG file of any type (defenderfix.reg was what I called the file, but any name will do, just make sure the file is a REG file, and not a TXT file)

5. next double click on the new file.

6. When you receive a message box asking "Are you sure you want to add the information in the REG file to the registry", choose "Yes"

7. You will receive another message box saying "Information in the REG file has been successfully entered into the registry", please click "OK" to confirm.

8. Restart the computer.