Windows Vista SP1

I have 4 computers that run Windows Vista. Two desktop computers and two laptop computers. All except for one run Windows Vista Ultimate. One of the laptops (a tablet PC) runs Windows Vista Business. For some reason, one of my desktops picked up on SP1 and offered to install it, to which I accepted. The other 3 however, have not said one thing via Windows Update about SP1. Even when I force a check for updates... nothing! Ahhhh, the mysteries of Microsoft.