As long as you've burnt the restore to factory disks or burnt to a usb flash drive (in case of a disaster and you can then restore to Win 8 at least) then see if you can install Windows 7 through Win 8. Boot to Win 8, go to desktop and insert the Win 7 DVD. I haven't done this but it might work. But don't do anything until you've got the restore to factory medium created.
Another way is to remove the drive, buy a new one ( a Seagate 500GB 2.5" Sata is only 38 squids) and install 7 on that. You can then either keep the old one as a backup 'as is' or buy a caddy, format it, and use as a new backup. Please remember to burn the recovery media first.