Another solution you might want to try which i also use on my laptop is, downloading a virtual machine like virtual box.....go to virtualbox.org then download the windows version on your windows 8.1 then install windows 7 in the virtualbox.
or
get an external hard drive, install windows 7 in it, then next time you boot up and want to boot into windows 7, with the hard drive plugged in, start up your computer and it boots into windows 7 or if you having troubles with that, just select the hard drive from your bios to boot up windows 7