When you try to start Windows Defender after you upgrade from Microsoft Windows XP to Windows Vista, you receive the following error message:

Application failed to initialize.


This problem may occur if you uninstall Windows Defender from the computer after you upgrade the operating system to Windows Vista.

Note If you receive the error message that is mentioned in the "Symptoms" sections but you have not uninstalled Windows Defender through Programs and Features, you may be receiving this error message because of a different issue. The steps in this article address only issues in which Windows Defender has been removed from the system.


To resolve this issue, use method 1. If method 1 does not resolve the issue, go to method 2.

Method 1: Use System Restore to restore Windows Vista

Use System Restore to return Windows Vista to a restore point that was created before this issue occurred. To do this, follow these steps:

  1. Remove all floppy disks, CDs, and DVDs from your computer. Then, restart the computer.

  2. Click Start Start button , click the arrow next to Lock, and then click Restart.

  3. Use one of the following methods, as appropriate:

    • If the computer has a single operating system installed, press and hold F8 as the computer restarts. You must press F8 before the Windows logo appears. If the Windows logo appears, wait for the Windows logon prompt to appear. Then, exit Windows and restart the computer.

    • If the computer has more than one operating system, use the arrow keys to highlight the operating system that you want to start in safe mode. Then, press F8.

  4. On the Advanced Boot Options screen, use the arrow keys to select Safe Mode, and then press ENTER.

  5. Log on to the computer by using a user account that has administrator rights.

  6. Click Start Start button , click All Programs, click Accessories, click System Tools, and then click System Restore.

  7. Follow the steps in the System Restore wizard to return Windows Vista to a previous restore point.

Method 2: Reinstall Windows Vista

For more information about how to perform an in-place upgrade of the Windows Vista operating system, click the following article number to view the article in the Microsoft Knowledge Base:

918884 How to install Windows Vista

Follow the steps that are listed in the "Upgrade Procedure" section to perform an in-place upgrade of Windows Vista.


Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

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