Use the windows update link in your start menu.
I had a similar problem but in my case I could click on the icon just fine. I'd double click on it and the window that shows the updates would pop up. But, when I clicked on the button to install them, the window would just disappear along with the tray icon. Eventually the tray icon would be back. I never did find an answer for it so I just updated via windows update. That worked fine and the icon would disappear until the next time there was updates available. Installing SP2
took care of the problem for me but I see you're already running it so that's really no help.
I'll have a look around and see what I can find. I didn't really spend a lot of time looking for a solution as the windows update work around was just fine for me. I still got the notifications and the shield icon dissappeared after using windows update so..... Maybe somebody else will have some suggestions for ya.