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1. Back up everything, of course. Make sure you have installation media for any programs you want to migrate.
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Go to the PC manufacturer's website and download all of the drivers for that particular PC. While some companies claim they won't guarantee their Win8 systems will have Win7 drivers available -- HP is particularly notorious in this regard -- in practice, I haven't heard of any Win8 Pro PC manufacturer that fails to provide compatible Win7 drivers. (If you know otherwise, please leave a note in the Comments section.)
2. Here's the tricky part: You need to find a "genuine" copy of Windows 7 Professional. If your company has a volume license, that's easy; the VL distribution kit includes all of the bits necessary to do the downgrade. Otherwise, you need to find a retail version of Windows 7 Professional or an OEM copy. (Hint: If you know someone with a TechNet or MSDN subscription, they can get it for you.) If you're downgrading a bunch of machines, you need only one copy of Windows 7 Professional. After all, Microsoft is selling you a license -- embodied in the product activation key -- not the bits.
3. Turn off Secure Boot. Most new Windows 8 Pro machines have UEFI with Secure Boot enabled. Fortunately, on every Windows 8 machine I've seen, it's relatively easy (but sometimes tiring) to turn off Secure Boot. Some computers let you get at the UEFI/BIOS settings during boot (typically by pressing Del, Esc, or F2), but you may need to get into UEFI through Windows 8. Here's how most Win8 machines work: Bring up the Charms menu, tap or click Settings, PC Settings, General, and choose Restart Now. When the computer comes back up for air, it'll show you an advanced options menu. Tap or click Troubleshoot, Advanced Options, UEFI Firmware Settings, then Restart. The computer restarts again, and you can see your UEFI settings. Change the Secure Boot setting from Enabled to Disabled. Save changes, and reboot your machine back to Windows 8.
4. Go through a normal Windows 7 installation. When you get to the part where you need a product key, type in the current Windows 8 product key (if you have one) or skip the key entirely. Install your downloaded drivers.
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5. Activate your newly up -- er, downgraded Windows 7 over the phone.
If you run both Win8 and Win7 in a dual-boot configuration, you need to pay for both; you can't "downgrade" Win8 to Win7 Professional and use both copies. You can run Win7 machines inside Hyper-V sessions in Windows 8 Pro, but problems can arise when you try to connect peripherals to your system and have them show up in the Win7 VM -- and, again, you have to pay for the Win7, you can't use a downgraded copy.