This is a very common question. The answer’s a bit buried, but it’s actually quite simple.
And for the most part, it applies not only to Windows 8, but Windows 10, Windows 7 and Windows Vista as well.
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The magic word? netplwiz
In Windows 8’s tiled Start screen, just start typing “netplwiz”:
Click the netplwiz icon when it appears.
In Windows 7 and Vista, type the Windows Key + R, or click Start > Run:
Type “netplwiz” and click OK.
This opens a User Accounts dialog:
First, click the account listed that you want to have automatically login and then uncheck the Users must enter a username and password to use this computer option:
Click OK and you’ll be asked to provide the password for the account. Once you’ve specified that, you’re done. The machine will now bypass the login prompt when rebooted and login the specified account automatically.
Auto-login is a security risk. Anyone could walk up to your computer, even if you had logged out or locked the screen, and reboot it, pulling the plug if necessary. On reboot, the machine would automatically login and they would have complete access.
Also remember that this is completely unrelated to any login that might be enforced by your screen saver. That’s a separate setting specifically for screen savers and actually has nothing to do with logging in to the PC. It’s simply a security setting applied when you dismiss a screen saver.