An OEM copy of Windows 8.1
The kids have moved on to 7 or 8.1 quite a while back, but my VMs are still running XP.
I'm not really looking forward to increasing all the VM disk sizes; are there any instructions people have successfully used for minimizing the disk consumption of W8? (I want to run Atmel Studio, or CCS, or Keil, or PSoC designer. I don't need a bunch of consumer-oriented apps with "live tiles.")