"how do i turn on windows defender?"?

I am using windows vista home premium. My computer is running not bad,pretty good I guess. Virus scans are turning out good. The only problem is I cant turn on windows defender,why i dont know. The only windows updates im not getting is for windows defender. I know or have an idea how to turn windows defender on but it just wont let me. This is the message im getting: Windows defender is turned off by group policy: Windows defender wont provide protection against harmful or potentially unwanted software and it wont send you alerts because it is off. To help protect your computer against harmful or potentially unwanted software,contact your system administrator to enable Windows Defender via group policy. I found a way to turn it on,clicked on the "turn on" button but it just froze on me. Any help or advice you can give me on this matter will be gratefully appreciate.Thanks all in advance!