You can try this fix - http://support.microsoft.com/default.aspx/kb/934562/en-us
If you have addidional Vista questions you can post here - http://windowshelp.microsoft.com/communities/newsgroups/en-us/default.mspx
If you have additional Defender questions you can post here - http://www.microsoft.com/communities/newsgroups/en-us/default.aspx?dg=microsoft.private.security.spyware.general