Engel

You might try this. Go to the main Windows Update window and
click on "Installed Updates" (on the left-hand side of the window, at the
bottom), and then remove "Update for Microsoft Windows (KB931099)" (only).
Reboot. Reboot again. Go back to Windows Update, and re-install the same
update, by clicking on the Check for Updates button.


This should essentially re-install Windows Defender on Vista.