In Server Core 2008 R1, this was a bit of a mess. Microsoft provided us with a script. We had to type:
Cscript c:\Windows\system32\scregedit.wsf /AU 4 to turn Automatic updates on.
To turn them back off we had to type Cscript c:\Windows\system32\scregedit.wsf /AU 0.
With R2, we have an easier method. Type sconfig and press Enter
Press 5 for Windows Update Settings.
Press A for Automatic or M for Manual.
You will receive the prompt above to let you know that you disabled Automatic updates or, you will see the one below if you turned them on.