No "Check for updates" in Windows Update

Hi --

On a Vista Home Premium SP2 PC, Automatic Updates is switched off (right now
on purpose). However, when I manually open the Windows Update applet, the
"Check for updates" option at the top of the list is grayed out and
inaccessible. So the PC is not pulling updates automatically, and I can't
pull updates manually.

Why? How do I reenable it? Note that I DO NOT want to turn on Automatic
Updates at this time.