Happy New Year!
I've had quite a few direct emails about this in the last few weeks, so it's probably worth posting as an FAQ. As you probably know, and as I have stated before, Hyper-V (codename "Viridian", and formerly known as Windows Server virtualization) requires assistance from hardware to operate correctly. Specifically, it requires four capabilities: An x64 CPU; VT (Intel) or AMD-V (AMD) hardware extensions; No eXecute (NX)/eXecute Disable (XD) and full BIOS support for hardware virtualization. It's 4 out of 4 required, nothing less. On Intel platforms, you can normally turn VT on and off in the BIOS, whereas on AMD platforms, AMD-V is on all the time (at least I have not come across a BIOS which contains an option to enable/disable AMD-V). NX & XD are usually always configurable, regardless of platform.
The difficult part is knowing where to turn these settings on and off. While I wish I could give an unified simple answer, there unfortunately isn't one as the BIOS configuration layout is down to the motherboard manufacturer. NX/XD will commonly be located under security settings, and VT will commonly be a processor option. One thing you should note is that if you do change VT to enabled, you should hard power cycle the machine - generally a Ctrl/Alt/Del or "Exit & Save" option is insufficient for the change to take effect.
As for full BIOS support - if you install the Hyper-V role and discover that the Hypervisor has not launched (you'll get an error message when starting a Virtual Machine) after enabling the hardware features, it is pretty much up to the motherboard manufacturer. My advise would always be to ensure you have the latest BIOS from the manufacturer before installing the role. While many, if not most, recent motherboards do have support, there really is no easy way short of directly contacting the manufacturer of determining if their motherboard has full support.