It always asks for Recovery Key when I boot to Windows, after I set up dual boot (Windows + Ubuntu)

1. To boot into Windows 

Press F12 to enter boot menu. Choose Windows.

 

2. To boot into Ubuntu

(1) Do NOT press F12. Simply wait, and choose 'Ubuntu'. 

Explanation: To boot into Windows, use Windows Boot Manager; to boot into Ubuntu, use GRUB. 

(2) Press F12 to enter boot menu. Choose Ubuntu.