Vista - Can you be sure that Windows software / security updates are in its place?

Windows Software and Security Updates: A restart is required or not?

A software update is what you need to take advantage of new fixes or features for a program or for any application in Windows. A security updates is your first defense in protecting your computer against known vulnerabilities in any affected software. Both type of update sometimes requires a restart before the update is placed properly but there are times, Windows has failed or has not place the installed software update or security updates for your system unless you will restart the computer. How is that possible?

Scenario: You installed a software update or security update but after installation, a reboot is not required. Not required because Windows did not prompt you to restart.
This means... the user is not sure whether the security update is in its place and protecting already the PC from known vulnerabilities that was fixed by installing the said security update.

Scenario: You installed a software update or security updates... Windows prompted you to restart the system to complete with security updates' installation. You followed the rules by restarting the PC. You ran Windows Update check again and found no more updates is needed for your PC. Are you done? Maybe yes or maybe not.

How can you be sure that Windows software and/or security updates are in place already?