Computers like the Intel “Mac Mini 2,1” were left behind with Snow Leopard (OS X 10.6.8) and Boot Camp 4. These restraints keep older machines from the benefits of current mac features, applications, and supported installs of Windows 8. However, these machines are well within the Microsoft system requirements for Windows 8.1 and installs are possible.

The older Core Duo Mac series cannot boot into 64-bit operating systems, and is excluded from modern versions of OS X, but Microsoft still supports 32-bit processors, giving many old Macs the opportunity to be re-purposed with a modern OS. I can confirm that if you use Boot Camp to install Windows 7 32-bit (no key needed) on a Core Duo Mac Mini, you can upgrade to Windows 8 32-bit from within Windows 7, and then do the same for Windows 8.1 (provided you have a license key for the final OS). Boot Camp 4 drivers work well for the 32-bit Windows 8.1, but I installed each one individually instead of using the BootCamp package ins …

Source: Mac OSX Hints