info about an installation of Vista update

Hi,

I click the start button of Vista and on the right is a red button with !
over it.
if I click over this button the install of the update start and turn off
the pc.
I had problems with my firewall not Vista firewall now resolved.
The windows update; my settings were warn me of new updates but not
download and install.
When I've checked in the status of my update settings were changed to :
download it but don't install it.
Now I think this could be a trojan.

How delete this installation that is ready to start if I click this button?

Thanks