An update is available for Windows Defender on Windows Vista. From Microsoft Knowledge Base Article 931099:
Windows Defender offers real time protection for your computer. Regardless of your Windows Operating System, if you need help with alert messages after scanning with Defender, click on over to the Security Tips & Talk Blog where you can learn "What do Windows Defender warnings mean?"
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.