First, ensure that you own a 64bit CPU with 64bit host OS (computer properties).
Then run Securable and VMWare Guest Check to confirm you can run 64 bit OS, so, to known your system is Virtualization (VT) capable.
Now you may come to a error like this one: "Software virtualization is incompatible with long mode on this platform. Disabling long mode. Without long mode support, the virtual machine will not be able to run 64 bit code. For more details see http://vmware.com/info?id=152."
This means that even though you may have VT, it is most likely, disabled.
If you're not sure if you have VT and/or it is enabled you should burn vt.iso and boot it, since the applications above will only tell you if your CPU is 64bit and VT exists somehow. This checker will tell you exactly what is the state of VT, which will be one of the following.
: This core does not support long mode CPU : This core does not support VT CPU : Feature control MSR is unlocked! CPU : VT is disabled in the feature control MSR! <-- I had this CPU : VT is enabled on this core.
So now that?
If you can find options in your BIOS that allow enabling the VT/Virtualization/VT-X, enabling them will be enough.
In my case, I have no options in the BIOS to enable VT, but there is a way to enable it even so!
Boot up a USB flash drive with some boot system from Windows 98 and place symcmos.exe and pedit.exe. The first grabs the CMOS and the second edits the VT flag on it. Here is the step-by-step procedure.
1) CREATE A BOOTABLE USB FLASH DRIVE WITH HP FLASH UTILITY
2) PLACE SYMCMOS.EXE AND PEDIT.EXE
Source: Enabling virtualization on a Sony Vaio (The Unofficial guide)
3) Reset BIOS to defaults, enable external boot devices, and configure USB flash as first boot device.
4) after boot, type "symcmos -v2 -lcmos.sav" and then "pedit cmos.sav"
5) look for 02D0 (for me its this entry, since its a Vaio VGN-FZ31M) and change it from 0000 to 0001 (this is a general activation method)
6) save with menu command alt+f and then exit pedit. the command line may crash, but we can reboot with no harm
7) upload the new cmos with command "symcmos -v2 -ucmos.sav"
Other resources Enable virtualization on VAIO laptop
UPDATE: On Windows 8 Developer Preview I had to choose "install OS after" instead of picking the ISO/DVD on the first screen, or I would get a "<productkey>" </productkey> error.
If you need to reinstall Windows Developer Preview or use the Reset functionality, you might be asked to enter this product key: 6RH4V-HNTWC-JQKG8-RFR3R-36498
If you’re running a server version of Windows Developer Preview, you can use this product key:
VmWare 5.5 rolled back the install everytime and it turned out to be issues with unsigned drivers. Booting with F8, option "disable driver signing" would allow the install to complete successful, throwing some warnings about the drivers. I think v5.5 worked fine in W2008 SP1 with its "experimental x64" option but now in SP2 I had to try with a later version of the product because it was making the host OS unstable.