Microsoft recently released Windows 10 to the public and an area of significant change is the Windows Update process.  Basically, security patch installation and associated reboots are required, and a faster update cycle will be enforced which includes delivery of new features along with those security patches.  As usual, many will voice their complaints but this is a direction that is now necessary and is becoming common practice in the industry.  With the slew of recent security announcements, vendors must be more agile in their patch abilities.  Firefox and other software vendors have recently implemented forced update procedures as well.  Regarding the new Windows 10 Update policies, Computer World has called it the “biggest-ever change” and a recent Forbes article states this is a “dangerous new direction.”

For individual users, updates will be installed then a prompt will be displayed to reboot. Eventually, the OS will schedule the reboot. Those reboot times can be controlled by the user (see screenshot below)

Schedule Option, in Settings>Update:
Windows Update Schedule Reboot Updates

 

Microsoft has also given new guidance to organizations who control their local environment’s update rollouts. Their advice to IT pros is to deploy security updates as soon as possible. IT pros should validate optional updates/hotfixes and then deploy them proactively. Baselines and convenience updates are the lowest priority but should be tested and deployed soon after release.

See advice from Microsoft in this Channel 9 Ignite video
Here is the summary graphic from that event:
Windows Update Microsoft Guidance for Organizations

 

If you absolutely do not want Microsoft to automatically update your OS, a potential workaround is to add all known Windows Update locations to your local hosts file. This will cause a Windows update error. If you then need to manually update, remove these from the hosts file:
download.windowsupdate.com
download.microsoft.com
update.microsoft.com
windowsupdate.com
windowsupdate.microsoft.com
ntservicepack.microsoft.com
wustat.windows.com

A note about the “defer updates” option:
Some Windows 10 editions let you defer upgrades to your PC. When you defer upgrades, new Windows features won’t be downloaded or installed for several months. Deferring upgrades doesn’t affect security updates. Note that deferring upgrades will prevent you from getting the latest Windows features as soon as they’re available.
From <http://www.bing.com/search?q=Defer+upgrades+in+Windows+10&filters=guid:%22a0eb9743-5670-4bad-858f-c815c00a2227%22+lang:%22en-us%22&form=S00028>