At what point during the install?
I have not yet tried 2012 on my 2850's, but according to a Google search, Execute Disable must be enabled in the BIOS and is only supported on the following processors:
Also, version A02 is the version that added support AND enabled this setting (may not be user-editable in the BIOS Setup), so make sure your BIOS is at least A02 - A07 would be strongly recommended (and possibly required).