It's not called a downgrade, its a reinstall...

The only way I know to remove Windows 8 or any installation of Windows is to run the setup from a windows disc or USB device. From there its just a matter of deleting (formatting) the hard drive that Windows 8 is installed on. Then the Windows 7 setup will install to the drive Windows 8 was on. However it is highly recommended you write down your motherboards model number and other hardware devices model numbers, if applicable. This way you can recover your drivers from the internet. Most new computers have the drivers stored on the C drive in a folder called drivers or similar. Usually on a computer newer than 2007 Windows 7 is able to install all the drivers for it, generic drivers, which can be updated using windows update or the hardware manufactures website. That's about the biggest issue you will encounter in a reinstall. Please note that a full format of the hard drive will result in a total loss of any information you have stored on your computer. Not just the operating system, but any personal data such as pictures, music, etc will be lost unless you have backed it up. All pre-installed software from the PC maker will also be lost. However this is actually a good thing since all the pre-installed bloatware will no longer be starting up and slowing down your boot time.

There is also the option of installing a dual boot setup. With this method you can keep both Windows 8 and install whatever operating system you like on a different partition of the same hard drive. This can be accomplished almost the same way as a regular install of Windows. The only thing different you have to do is setup a secondary partition using Windows computer management. This windows application can be found under the administrative tools menu on the start menu (in windows 8 I have no idea where it is). If you can find it, then you can create a new partition on the C drive which uses the free space on your hard drive so be careful and read your total amount of free space before setting the new partition size. Once you have created the new partition, insert your install media (DVD/USB) and reboot the PC. Then press F8 or whatever key that allows you to manually choose your bootup device. Then choose the DVD drive or USB device that contains your OS install media. The computer will enter setup and you will see two hard drives this time. Choose the drive that you just created when you partitioned the hard drive, as the install location for windows 7. Also, be sure to leave the other drive alone as that will contain your Windows 8 install and all related programs/media. Install the operating system and answer the questions it ask you along the way. If my rough explanation is not clear enough for you to understand then refer to these Microsoft installation guides.

http://windows.microsoft.com/en-US/windows7/Installing-and-reinstalling-Windows-7

or for dual booting try,

http://windows.microsoft.com/is-IS/windows-vista/Install-more-than-one-operating-system-multiboot

I've tried Windows 8, and its nice looking and all. Though I ran it in a dual boot situation on a laptop I have. That way when I was done checking it out, and found it not to be the right operating system choice for me. All I had to do was erase the Windows 8 partition after booting to the Windows 7 install that was already there.

I completely understand why you would want to change your operating system. Windows 8 is for touchscreen computers and most desktop users will not be able to utilize the full functionality of Windows 8 unless they are willing to invest in the hardware. In a few more years all new computers sold will have some kind of touch interface and then Windows 8 may find more of an audience.