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On the start screen, please hold Ctrl + Alt+ Del.
Hold shift key, click power button and select Restart.
Click Troubleshoot when the following screen appears
On Troubleshoot screen, plz click Advanced options
On Advanced options screen, plz click Startup Settings
On Startup Settings screen, plz click Restart
Then you can see many startup options, you can press F4, F5, or F6 to boot in the wanted safe mode.
I don't know why Windows 8 can be so janky to get to start in safe mode, but it sure can be if your machine is not booting up normally. Essentially, when you power up and the start sequence begins, hold down SHIFT and tap the F8 key like an OCD madman.
This slide show on About.com will walk you through it. Basically, though, you:
Open Advanced Startup Options
Choose Advanced Options
Choose Startup Settings
Touch or Click Restart Button
After the computer restarts...
Choose a Windows 8 Safe Mode Option
Wait While Windows 8 Launches