Hi Bob,
There are a few different ways to go about doing this. If you have the installation CD, or a USB stick with Windows 8 or later on it, you can boot of of said device and in the installer, select the "Repair my computer" option, clicking on Troubleshoot -> Advanced Options -> Startup settings -> Restart. The screen will change and have numbers next to the options on the screen, to enter safe mode, press the corresponding key on your keyboard.
I hope that helps!
David