WARNING: I have not evaluated the articles listed below for accuracy or usability. Always create a recovery usb stick and properly image and back up your computer before attempting any of these processes. You will be changing and creating partitions on your hard drive and any error could result in trashing your existing Windows installation. Also, always read the COMMENTS in any blog post BEFORE following the steps in the post, as there may be corrections or issues!
Creating Dual Boot on a NON UEFI PC:
Windows 10
How To Install Ubuntu Linux Alongside Windows 10 (PC is Standard BIOS, non-UEFI)
Windows 8.1
Linux and Windows 8 Interoperability
How To Install Ubuntu Linux Alongside Windows 10 (PC is Standard BIOS, non-UEFI)
Windows 8.1
Linux and Windows 8 Interoperability
Creating Dual Boot on a newer UEFI PC:
Dual Booting Windows and Linux - What You Need to Know
Dual-Boot Ubuntu 15.04/14.10 And Windows 10/8.1/8: Step By Step Tutorial With Screenshots
Seven ways to set up multi-booting with Windows 8 and Linux
YouTube: Installing Linux on Windows 8 UEFI Secure Boot System