As Windows starts the following message appears: ?Windows Defender. Application failed to initialize: 0×800106ba. A problem caused Windows Defender Service to stop. To start the service, restart your computer or search Help and Support on how to start a service manually”. How can I prevent this message from appearing on startup” The computer is running Windows XP.

This is a common error which other users have been experiencing, and Microsoft have outlined a procedure for resolving the error code (0×800106ba) which you provided: support.microsoft.com/kb/931849 . While this Knowledge Base article does not quote the exact error message which is appearing on your computer, the steps provided to resolve the problem are the most common suggestions to resolve your instance of the problem and therefore should be quite useful.