The "System Builder" version of Windows 8.1 is for installation on a new computer (with no operating system installed) or on a computer that is NOT currently running Windows 8, Windows 7, Vista, or XP. This product ("System Builder") is NOT for upgrading; if you are upgrading from a previous version of Windows, then get the upgrade package.

This is the "OEM" version of Windows 8.1: this means you can't get technical support from Microsoft; it also means you can activate it on one (and only one) computer (ever). If you want technical support and/or the potential to transfer the license to a new computer, then look at the "full" version (Microsoft Windows 8.1 - Full Version).

This is NOT the "Professional" version of Windows 8.1. Make sure you are selecting the version you need/want. To learn more, just perform a web... Read more

I was building a new PC, and I wanted this OS because it is more modern than windows 7, which is from 2009, and is less demanding, meaning more of my hardware can be focused on gaming.
Here are the pros and cons
Pros:
-as said previously, better optimization for new hardware. This OS will use up to 8 threads, so it puts less stress on AMD 8 core processors and hyperthreaded quad cores. It also just flat out runs better with newer hardware. I have a quad core non hyperthreaded processor (i5-4570), and it gets better benchmarks than on windows 7.

Cons:
-UI. This is the most basic gripe that a lot of people complain about, but it really isn't very hard to fix. I just installed classicshell, and now I can use my computer like it's windows 7. That said, seeing how easy of a fix this was, it wouldn't have been hard at all for microsoft just to put an option in control panel to have a start button.
-No administrator account. Even if you set your account as... Read more

Product includes DVD and the key. You can only install this on one computer. Installing it even inside a Virtual Machine (such as Parallels on a iMAC) will require a reactivation call to Microsoft, deactivating the other, previous install. I placed this on a dual boot iMac 2012 bootcamp Mountain Lion allocating 64 gig of space out of its 1000 gig fusion drove. You can only use the 64 bit version of Windows 8.1 (as this is).

The install on my 2012 iMac went well enough if bootcamp instructions are followed, I did not have any problems losing the keyboard even though they were wireless. Programs are to be installed on an external drive to save internal hard drive space. The drivers for USB 3 are included in Windows 8.1 so there should be no issues installing this on a 2012 Mac, my main reason for getting this over Windows 7. Some instructions for Windows 7 under bootcamp had a fair amount of user comments relating to having to obtain the USB 3 drivers before Windows 7... Read more