Update Off; Cannot Turn On

I recently upgraded from XP SP2 to Windows Vista Ultimate. Previously, I had
auto update off. However, when I load the Windows Update to change options,
the box associated with Turn Off auto update is checked and greyed out. I
have tried to check the other boxes and the selection is accepted without
changing the bottom box which indicates that auto update is off. When I
reload the update options, the update box I checked is "unchecked" and the
box for "turning off auto update" is checked and greyed out. I do not receive
any error messages throughout the process. I suspect I need to make a change
to my registry to allow me to turn on auto update. Can anyone help me here?
Thanks in advance ...