First use your finger to make a sliding motion from bottom to top on the illuminated screen in Modern UI, then click once All Applications. If you are using a mouse, please right click once All Applications.
(http://support.microsoft.com/Library/Images/2830118.png)
Then on the command command prompt character use your finger to slide from top to bottom, and click once more Executable by system administrator. If you are using a mouse, click on the command prompt character and right click, then click once more Executable by system administrator.
(http://support.microsoft.com/Library/Images/2830119.png)
If there is user account control, please click once yes.
(http://support.microsoft.com/Library/Images/2830120.png)
Click to input Bcdedit /set {bootmgr} displaybootmenu yes,when completed press the Enter key on the keyboard. If you need to cancel the prompt to enter options at startup, simply change Yes to No.
(http://support.microsoft.com/Library/Images/2830121.png)
Once completed, please restart your computer, then press F8 to enter setup screen upon startup, then press F4 or 4 to enter safemode.
(http://support.microsoft.com/Library/Images/2809483.png)
Source: http://support.microsoft.com/kb/2809468