This Might Serve as an answer :
The OS Upgrades during installation puts a message “Do not shut down your machine”. If I shut it down the machine become unusable and I have to reinstall the OS?
First thing The
Update Might fail if you shutdown the machine, You can read this section for updating os ,If your update is optional update the system wont crash.
If the warning is just a caution with no harm done if I power down the machine, how does the update process do this?
It's Not Just a Warning, Read this question For Reasons ,Apart From it if you wish to see which service is initializing the update
From William hillson's answer :
Worst-case usual scenario - It is in the middle of updating a system file / half way through the process and a restart means it is
missing as it is does not exist at the moment. However, Windows Vista
is quite good at repairing itself and may boot into the recovery mode
and roll back the update for you, so you will be out of action for a
short while (probably under 30 minutes)
Utter worst case unusual scenario, it is updating a boot file (but not sure how many updates do this) and restarting will give the
message "Operating System Not Found" or similar. In which case you
will need to put in the original disk and do a startup repair or at
very worst case, a reinstall of Windows.