You can create a file with your data on it with Windows Easy Transfer. I would strongly recommend that you image the drive first to something other then the boot drive. That way you can go back to Windows 8 if you want to. Then use the transfer utility to make a copy of your data/user folder onto something other then the boot drive. Then you will need to install Windows 7 over Windows 8, if the installer will allow you. If it won't let you install over it, you will need to format the partition then install it. The later solution would be my preference, since all the information on the system about software you have installed would be lost. Thus no way to uninstall the software with Windows "Programs and Features" utility.
Most pre-built systems these days have a recovery partition that can get you back to a factory installed stated (it will wipe any programs/data you have on the system that weren't on there from the factory). If that is the case with your system, you could skip the image, but I would still strongly suggest you do it anyway. This would also give you a complete backup so you could get anything you may have missed with the transfer utility.
This isn't for the feint of heart. If you aren't confident in what you are doing, I would suggest against it. There are some options to get back to something more like the old Windows interface that are out there. Stardock (
www.stardock.com) is one I know of. I am still on Windows 7, so I have no experience with any such software and cannot endorse any. Just pointing you to one that I am award of.