can't turn off auto update :(

I'm logged in as the Administrator for my computer, but when I go to the Automatic Updates menu, I'm locked out of all the options, and the "automatic download but manual install" is automatically checked. Is there a reason for this? And can I change it? I'd rather do manual updates through the website so I can choose what I get and what I don't, and whenever the update client runs it hogs all my system's resources and makes everything soooo slow! Please help. Help is very much appreciated. Thanks.