It's relatively straightforward to downgrade from Windows 8 Pro to Windows 7, but if you are using plain Windows 8 you'll need to buy and install a Windows 7 key. That's not a cheap option but Microsoft doesn't offer downgrade rights with the consumer version of Windows 8. It's Windows 8 Pro or bust, in this respect.
Other restrictions include the fact that you can downgrade Windows 8 Pro only to Windows 7 Pro or Windows Vista Business. (In essence Microsoft understands why businesses with a fleet of Windows 7 PCs may wish to downgrade new PCs so that all their systems use the same OS, but it doesn't want to make it easy for consumers to downgrade.)
Here we explain how to downgrade from Windows 8 Pro to Windows 7 Professional. To downgrade a consumer Windows 8 PC you must buy a Windows 7 DVD, but otherwise the steps are identical. Of course, if you are a consumer and you buy a Windows 7 DVD you may choose instead to install Windows 7 on a partition and retain Windows 8.
How to downgrade Windows 8 Pro to Windows 7: before you start
You'll needa Windows 7 installation disc. You can buy it, of course, but with downgrade rights you just need an Windows 7 DVD or USB stick and your Windows 8 Pro activation key. If you're moving from Windows 8 Pro to Windows 7 Professional you won't need a new license key, but you will have to call Microsoft to obtain a single-use activation code after Windows 7 is installed - attempting to activate Windows 7 online will bring up a local Activation Support phone number.
You also need to make sure you have all the important drivers. In Windows 8 open the Device Manager (hit Windows and X, then choose Device Manager). Now expand each device until there's a list of all the components, then write them down or take screenshots. You need the manufacturer and model numbers for everything, but the three most critical components are display adaptor, network adaptor and pointing device (if you use anything other than an ordinary mouse).
Now you need drivers for every component. Search on the web for the support pages of each component and download them. Then and store them on a USB drive.
Next you need to back up all the non-Windows software you have installed on your Windows 8 Pro PC. The chances are that you haven't installed much given that you wish to downgrade a PC that is, presumably, new. Also remember to back up all your files. The downgrade is destructive - you'll lose everything that was on your PC.
You want to have all of your files stored externally, as well as the discs and codes for all your software. And remember that you won't be able to install Windows 8 apps on your new system. You'll also need a Windows 7 disc and an activation key.
How to downgrade Windows 8 Pro to Windows 7: the install
Boot your PC from the Windows 7 disc - you will probably have to pop the disc in the drive and hit F2 or another key during the boot up process.
To be honest, that's all the heavy lifting done. The Windows 7 installer will ask you for details such as your geographical location. Follow the prompts. The system will reboot several times until Windows 7 is installed.
Now you need to reinstall all the drivers you stored on the USB or CD. Then reinstall the software you set aside, and load up your files.
You now have a Windows 7 PC.