Since Windows 8, everything is now Metro, not just the start menu, but the boot loader too! Fortunately, there is a command to disable the Metro boot loader and enable the legacy one.
To do so, you will need to open the command line utility as administrator and use the following command :
bcdedit /set {default} bootmenupolicy legacy
To switch back to the Metro boot loader, use the following command instead :
bcdedit /set {default} bootmenupolicy standard