Good news, it IS now possible to install Windows Server 2012 R2 on a PE840.
First, create a bootable USB thumb drive from the Windows 2012 R2 ISO using Microsoft's free Windows USB/DVD download tool available at:
Then, set the boot order in the PE840 BIOS to boot from this device first.
Now, boot from the thumb drive and Windows 2012 R2 installs until it says it needs to reboot.
Remove the thumb drive or change the BIOS boot order so that it doesn't boot from the thumb drive again.
Windows 2012 will boot up the 2nd part the setup and complete it successfully.