If you are able to get into windows click the Start button > Power. Holding down the Shift button click Restart. In the menu that comes up go to Troubleshoot > Advanced options > Startup Settings > Restart. Once machine reboots you'll get the startup menu select Enable Safe Mode (/with Networking).
If you're not able to get into windows you need to use a recovery drive, if you have that restart your PC and press F8 when it starts up, this will bring you to the Advanced Boot Options where you can select to enter safe mode.