Dustin G mentioned this in a comment but I thought it merited a proper answer.
As has been said in comments, there are some updates the MS determine to be so important that they will be installed regardless, giving you no option to preview them.
What makes this irksome is that it will accompany the install with an auto reboot, which may may interfere with long-running e.g. overnight tasks.
if the windows version is suitable (eg Pro, Enterprise etc...) then you can prevent the auto reboot through Group Policy. (this won't stop the install, just present you with the standard reboot reminder dialog that you get when you manually run updates, rather than one that has a countdown timer to do it anyway)
open the local group policy editor:
Win 7: type gpedit.msc into the search box on the start menu
Vista: start->run-> type gpedit.msc
navigate to the following setting:
Local Computer Policy->Administrative Templates->Windows Components->Windows Update->No Auto-Restart with logged on users for scheduled automatic updates installations
double click it to open it up and select "Enable" to prevent the timeout of the restart dialog.
If you don't have access to gpedit.msc due to your Windows SKU, then create the registry entry directly: (create any keys that don't already exist)
create a DWORD value called NoAutoRebootWithLoggedOnUsers and set the value to 1
64bit: same as 32bit but use
for the key path