When launching the Windows Phone Emulator for debugging, I got the message “The Windows Phone Emulator wasn’t able to set some properties on the virtual machine. Couldn’t change Memory of the virtual machine”. I got this both on a Windows 8 machine (non-virtualized) and on Parallels on a Mac. It took a while to find a solution for Parallels. I had followed all the instructions they provided here. The emulator would start, display the error message, and close as soon as I clicked OK.

Then I noticed that I could expand the Processor node in the Hyper-V settings. Under NUMA is a place to set the maximum number of processors. I reduced it from 2 to 1. (I had already reduced the Processor setting from 2 to 1).

I clicked Apply and OK, restarted Visual Studio, and haven’t had the problem since. While this may not solve your particular configuration issue, I suggest  you give it a try if you’re having a similar problem.