Please note:  These instructions were tested on an HP model PC running Windows 8.  The following steps will vary depending on your PC manufacturer, model, and your build of Windows (8, 8.1, etc).

Step 1)  Inside Windows 8, navigate to the Charms Bar hidden to the right of the screen.

Step 2)  Select Settings, then Change PC Settings at the bottom.
Step 3)  Click the General tab.

Step 4)  Scroll to the bottom of that menu and click Restart Now under Advanced Startup.

Step 5)  Another set of options will appear.  Click the Troubleshoot option.

Step 6)  Click Advanced Options.

Step 7) Click UEFI Firmware Settings.

Step 8) Click the Restart button.

Step 9)  When the boot-up menu appears, select the Computer Setup option.

Step 10)  Navigate to the Security tab at the top, then select Secure Boot Configuration

Step 11)  A warning may pop up.  Simply choose Accept.

Step 12)  Navigate to the Legacy Support option and make sure it is Enabled.

Step 13) Navigate to the Secure Boot option and make sure it is Disabled.

These two steps are the main goal across most (if not all) systems.

Step 14)  Navigate to the File tab at the top and select Save Changes & Exit.  When asked if you're sure, click Yes.

Step 15)  At this point, if Windows 8 starts up normally, just repeat steps 1 through 8 above.

Step 16)  When the boot-up menu appears, select the Boot Menu option.

Step 17)  Select your CD/DVD-ROM drive.  Make sure you are clicking on the bottom-most item in the CD/DVD-ROM category as shown in the below image.

You should now be booting from the CD.  If these instructions did not match what you saw on your end, you will need to contact your manufacturer to get the proper instructions.  As mentioned before, this setup will vary depending on the manufacturer, model, and version of Windows you have.