As Tech fiends, we often find ourselves desiring the impossible. This is exactly what let me into attempting to put Win 8.1 on an iMac (late 2009). According to the Apple website, this shouldn't be possible because it doesn't support the latest version of bootcamp.
Okay, so here's what we need to think about when attempting this sort of thing. Will the hardware support 8.1? Of course! Is that even a question? What are our limitations? Software: drivers, versions, etc.
I tried a number of things first that ultimately didn't work. I won't bother listing them, instead I'll just explain what ultimately did work for me.
What you'll need:
1. Windows 8.1 .iso disc and license key.
2. Windows 7 .iso disc (no key necessary).
3. Wired keyboard/mouse.
Start the bootcamp assistant utility. Insert the Windows 7 .iso disc. Follow the on screen instructions (disable the updates check mark). Select you partition size and the computer should automatically reboot into the Win 7 disc. While it's rebooting, hold down the Option key. When the boot options show up, eject the Win 7 disc and insert the Win 8.1 disc. Boot into the Win 8.1 disc and follow the on screen instructions. When prompted to select a partition, select the Bootcamp partition (you may need to Format it first), and press continue. When you get into Windows, perform all the updates you can. Go into device manager and see if there are any unknown devices (the only one I found was the iSight was not installed, but the camera still worked fine).
And there you have it! Pretty simple, actually. In order to switch boots, you just hold down option while the computer is turning on and there you can select which OS to boot into. Good luck :)
