Can you help out with Window Defender?

I want to turn on my Windows Defender, but it won't let me. I keep getting this message: "Windows Defender is turned off by group policy. Windows Defender won't provide protection against harmful or potentially unwanted software and it won't send you alerts because it is off. To protect your... show more I want to turn on my Windows Defender, but it won't let me.

I keep getting this message:
"Windows Defender is turned off by group policy. Windows Defender won't provide protection against harmful or potentially unwanted software and it won't send you alerts because it is off. To protect your computer against harmful or potentially unwanted software, contact your system administrator to enable Windows Defender via group policy"

I'm the administrator, and I don't remember disabling or switching off Windows Defender and wish someone could help me enable it... Can anyone help?

I'm on Windows Vista Home Premium Service Pack 2
64-bit operating system
3 answers 3