I haven't had any auto-update problems either. You might try what I did to
get updates to work correctly. Download the Windows Install Clean Up utility
from Microsoft
http://support.microsoft.com/default.aspx?kbid=290301.
Uninstall Windows Defender using the Control Panel Add/Remove software
window. Run the Windows Install Clean Up utility and remove Windows Defender
(If listed) and Windows Defender Signatures. Reboot and reinstall Windows
Defender, but do not take the check for updates choice from the install.
Instead run Windows Defender and select "About Windows Defender" from the
Help menu and use the "Check for Updates" button from the "About Widows
Defender" window. It took me several misfires before I came to this solution.
I have seen a cautionary note about using the clean up utility that said you
might not receive Microsoft Office updates until a new update comes out. If
you have Microsoft Office I would suggest you make sure you have all the
updates before you try this procedure.