Update, May 15: With the Windows 10 Creators Update, Microsoft has largely addressed the forced updates that often resulted in lost work. And, while the the recent WannaCry ransomware does not (thus far) appear to affect Windows 10, you need to make sure your PC is kept up-to-date with security patches to avoid exactly those sort of attacks. To that end, consider the information below to be out of date, with a more thorough update to come.
As a general rule, an up-to-date operating system is a secure operating system, and some people are terrible at updating their operating systems.
That's probably why Microsoft has -- controversially -- decided to make Windows 10 an automatically-updating OS. Windows 10 automatically checks for, downloads and installs new updates to your PC -- whether you like it or not. This new feature is actually pretty convenient for most users, but not everyone wants their operating system updated on Microsoft's schedule. (And it's one of the reasons you might not want to upgrade to Windows 10 just yet.)
If you're running a professional version of Windows 10 (Professional, Enterprise or Education), you actually can disable automatic updates using the Group Policy editor. But if you -- like most people -- are using Windows 10 Home, you'll need to use workarounds to stave off Windows 10's aggressive automatic updates.
Install updates manually
The most obvious -- albeit laborious -- solution is to install updates at times when it's convenient for you.
Schedule restart times
If your main issue with Windows 10's automatic updates is the fact that your PC restarts at inopportune times, you're in luck: Windows 10 allows you to schedule restarts for a specific time. When a new update has been downloaded and installed, Windows 10 will prompt you to schedule a time for your PC to restart.
A small warning: Microsoft may be scrapping this ultra-convenient feature in the Windows 10 Anniversary Update. In the latest Insider Preview build (Build 14342), you can no longer schedule restarts. Instead, you have the option to set up "active hours" -- a 10-hour time period during which your computer will not be restarted.
Meter your Internet connection
The easiest way to prevent Windows 10 from automatically downloading updates is to use a metered Internet connection -- one of the features of limiting your data is that Windows 10 won't force-download OS updates. Some networks, such as cell networks, are metered by default, but you can also meter your Wi-Fi network by going to Settings > Network & Internet > Wi-Fi > Advanced options and turning Set metered connection on.
You can't meter your Ethernet connection through the Settings menu -- Microsoft assumes your Ethernet connection allows you unlimited data, apparently. The only way to set your Ethernet connection to metered is to perform a registry edit -- it's a little complicated, but here's how to do it.
You can still download updates on a metered connection, you'll just have to do it manually by going to Settings > Windows Update > Download.
Disable automatic driver updates
While you can't really stop Windows 10 from automatically updating the OS, you can stop it from automatically updating your device drivers. To do this, right-click the Start button and click Control Panel.
In the Control Panel, go to System and click Advanced system settings to open the System Properties window.
Open the Hardware tab and click Device Installation Settings.
When prompted with this message, click No and click Save Changes.