In the past, it was just a matter of booting the system from your CD/DVD or USB drive loaded with your preferred Linux distribution and bob's your uncle. The problem now is that Windows 8 certified PCs need to comply with "Fast Boot", for taking care of hardware initialization and setup very quickly and this now commonly means not initializing USB devices at boot time. With the USB devices not being initialized until the operating system starts, and many PCs today shipping without CD/DVD drives, it can be difficult to boot a Linux installer. Obviously without USB being initialized, you can't boot from your USB stick.
Microsoft does offer a special reboot option to access the firmware menu at which point the hardware is fully initialized, including USB, but that option isn't offered during the Windows setup stage prior to accepting Microsoft's End User License Agreement. So even if you don't agree with Microsoft's EULA and just want to wipe the disk to install your USB-based distribution, with Windows 8 it can now be a difficult task.
Matthew Garrett, after dealing with Microsoft Windows 8 Secure Boot challenges for numerous months, is now dealing with this latest PC problem. He's written about this issue on his blog.