Vista - Unable to turn on Windows Defender in Security Center

Hello out there.

I'm having some issues with the Malware Protection in the Security Center.
I can't turn on Windows Defender in the Security Center. When I try to
activate it, he searches for any available update. Once he detects he is up
to date, a warning message window pops-up stating that no updates are
available. Once I close this alert window, Windows Defender remains off.

I'm continuosly getting alerts from the Security Center. And I know that
this program is up and running.

Any idea? Thanks in advance