I have recently had to update a couple of Windows computers that hadn't been updated for a while (Windows 7 & 8). There seems to be a bit of a knack required, so I thought I would share my experience and knowledge gained.
Step 1: turn on the computer and leave it for a couple of hours. You may see a notification that Windows is downloading updates. It will probably stall at %18 or a similar figure.
Step 2: Reboot the computer. You may see an option to restart and install updates or you may not. You will see notifications about updates being installed while shutting down and while restarting. This step could take 9-10 hours.
Step 3: Repeat steps 1 and 2. Yes, this could take another 9-10 hours.
Your computer is now up-to-date. Enjoy! (Until next week, then go back to step 1.)