An update is available for Windows Defender on Windows Vista
http://support.microsoft.com/kb/931099

An update for Windows Defender on Windows Vista has been released. The
update corrects a problem in the functionality of Windows Defender malware
sample submission. This update applies only to Windows Defender version
1.1.1505.0 on Windows Vista. This problem does not occur in Windows Defender
version 1.1.1592.0. This problem also does not occur in later versions of
Windows Defender on Windows XP.

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Regards,
Donna Buenaventura
Windows Security MVP
w: http://cou.dozleng.com
b: http://msmvps.com/donna