TIP: PCWorld's sister publication Computerworld points out that you may have to disable the new Windows 8 secure boot feature in your Windows 8 machine's BIOS settings. To do this go to the Charms Bar in Windows 8 and select Settings > Change PC Settings > General > Advanced Start Up. On the next screen select Troubleshoot > Advanced Options > UEFI Firmware Settings (this option is available only on new Windows 8 machines).
Once you're set to downgrade, just pop in your Windows 7/Vista installation disc or USB drive and install the system as you normally would. When prompted for a product key, use the Windows 7/Vista key that came with the older OS' installation discs and not the newer Windows 8 key; however, you will need your Windows 8 key after Windows 7 is installed.
Regular online activation for Windows 7/Vista will fail because you used a product key that is already in use or was in use at a prior date on a different machine. So now you have to activate your Windows 7 downgrade installation by telephone.
Your computer should automatically alert you that online activation failed. If it doesn't you can manually activate your machine in Windows 7 by clicking Start > Control Panel. In the search box of the Control Panel window type activate and hit enter. Click the link to Activate Windows. When you try to activate online, the process will fail and you should see a window with a bunch of empty text entry boxes as well as a phone number for the Microsoft helpline.
During the helpline call you may have to speak with a customer representative. Just explain the situation and have your Windows 8 Pro product key available to prove your PC has downgrade rights. You will then be given a single-use activation code (usually it's annoyingly long) to enter into the activation window. Once the code is entered and confirmed, you're all done.
If you have more than one Windows 8 PC that you need to downgrade, Microsoft says you can just use the same Windows 7/Vista installation media on each machine.
Downgrading from Windows 8 Pro to Windows 7 Professional or Windows Vista Business is not a simple process, but at least the option is there if you really need it.
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