Windows Vista's Defender stopped working!

I have a laptop with Windows Vista Home Basic and recently Windows Defender
stopped working and keep saying:

"Application failed to initialize: 0x800106ba. A Problem caused Windows
Defender to stop. To start the service, restart your computer or search Help
and Support on how to start a service manually."

I have also tried to download windows defender from microsoft to reinstall
it but it will keep on saying windows defender already comes with vista and
you do not need to install it.

I have also tried going into the services and try to start it manually and
this is what it say:

"Windows could not start the Windows Defender service on Local Computer
Error 0x80096004: The signature of the certificate cannot be verified"

any help please?