I would start by backing up the windows 8 drivers. You should find them in c:\windows\system32\drivers. Just copy them straight over to a flash drive.
Next, make a backup of your factory image, just in case you change your mind or have other issues. The software to do that is usually part of the preinstalled software package and should allow you to create disks or use a USB drive. I would also use a key recovery program to recover the windows 8 key from the bios and save it to a text file.
Then install windows 7. Grabs the drivers off your manufacturers website and install them.