first go into the bios and see if you can ditch secure boot!

Provided this is not a laptop or Surface tablet we are talking about? Even so with most you will be able to ditch secure boot.

If you have a legit copy of windows 7 go into the bios ..still usually accessed by hitting delete during the POST (power on self test). Some companies like Dell will use other bios access methods but it is all documented.
In the BIOS there will be an option to disable secure boot, which is in reality not a real security issue or source of pc infections. Microsoft just made a stink with vendors and they all jumped on board so that it is now a little harder to dual boot computers off the shelf. (slows down the adoption of Ubuntu and Mint Linux)

You can then remove the existing windows drive put it aside if you want to keep windows 8 and resell the computer with it later. For cheap you can pick up another drive and just put windows 7 on it... the manufacturer most likely has alternate windows7 drivers for the hardware on their web site. Most windows 8 motherboards are exactly the same as the last generation windows 7 devices and the hardware drivers should not be problematic.

I would download and stick a copy of Mint linux or Knoppix linux on a thumb drive and try it out in live mode without having the secure boot get in the way to see how old your hardware really is. If all the devices work then chances are Windows 7 will run without issues. Again just unplug the windows 8 drive and you should not have trouble.

It is mandated that you can turn off secure boot and by doing so you will not have issues installing either windows 7 or running a test session of any version of Linux.

The whole secure boot nonsense is just a ploy to stop people from down grading to 7 or trying out Linux, don't let the shills on the net fool you, having secure boot turned off will not compromise your security or the functionality of your system!