Latest installment of the series "this took longer than it should have to sort out", in the hope it may be useful to someone else.

My computer has two E5520 Intel processors (i.e. eight cores), and all of my development happens on a number of VirtualBox machines. Today, I tried assigning more cores and memory to a Windows 7 guest, and it failed to start up with error VERR_VMX_MSR_LOCKED_OR_DISABLED - the log gives a little more information:

00:00:01.418 HWACCM: No VT-x or AMD-V CPU extension found. Reason VERR_VMX_MSR_LOCKED_OR_DISABLED