I've got a MBP8,1 running El Capitan (10.11.2). Until recently, I had a perfectly serviceable Win7 installation via Bootcamp. A few days ago, I decided to clean-install Win10 to replace Win7. Since then, I've been chasing my tail.
My previously functioning Win10 installation DVD wouldn't work; I kept getting an error about "a needed file may be missing" or something similar. I tried installing via VirtualBox and get the same issue. Tried installing on the Power Mac 3,1 under my desk and get the same problem (on VirtualBox - haven't tried Bootcamp). So I tossed the installation DVD and downloaded a brand-new Win10 ISO from Microsoft.
I'm having the same issue with the new install media (also tried via USB - no dice). Several times, Bootcamp Assistant was unable to create a new partition. Ran Disk Utility, repaired permissions, etc. Still having the same problem.
I backed up my hard drive, plugged the MBP into the MP and booted into Target Disk Mode, and used the MP to format MBP drive (writing zeros over all sectors - took freaking forever), then reinstalled OS X.
I'm currently downloading VMWare and am going to try it that way.
EDIT: No dice under VMWare either. Same "missing file" nonsense. Also tried installing Windows Server 2012 under VirtualBox on the Mac Pro. Hangs at 92%. I guess I'll try that one on the MBP under VMWare, but I have zero faith it's going to work. Something is broken somewhere.
EDIT2: Win Server 2012 installed via VMWare on the MBP. Still no idea what the problem is everywhere else.