I was unable to install Win 8.1 Pro RTM 64-bit under 4.2.18.
I kept getting a stop saying I needed to enable NX mode in my bios.
Finally, I checked to see how I got Win 8 Pro RTM installed and found I'd installed the 32-bit disc.
Eureka, I tried the 32-bit Win 8.1 Pro RTM and it installed just fine.
I understand there's a command one can run that will enable the 64-bit package to install but I found out after I'd installed the 32-bit.