hi
i am running vista home premium and everytime i try to switch in windows defender i keep getting the message
WINDOWS DEFENDER IS TURNED OFF BY GROUP POLICY
CONTACT YOUR SYSTEM ADMINISTRATOR TO ENABLE WINDOWS DEFENDER THROUGH GROUP POLICY
i didn't think vista home premium had group policy and if it does i can't find it
any ideas
cheers

http://technet2.microsoft.com/WindowsVista/en/library/957b0b80-0710-46e8-86d8-2509f993f1bd1033.mspx?mfr=true
Read this article.
Pay attention to this part:
To Locate the Group Policy Settings for Windows Defender
It may solve your problem. Good luck.