Having had issues during the last few months regarding installing an ESX4 inside an ESX4, I cam across a solution which might have resolved my problem happily as ever, I have been fighting to get my ESX4 box to run on an other Hypervisor and this wasn’t that easy.

This is written by  jmattson from the VMware Cummunity and I would really thank him for all this information,

Feel free to comment in any case,

“Intel first introduced VT-x on late-model P4 processors (codenamed Paxville). Many Core, Core 2, Core i7 and Core i5 processors are VT-capable (as are their Xeon-branded counterparts). However, some skus of these models are not VT-capable. (For example, the Q8200 Intel Core 2 Quad Processor is not VT-capable). You can check to see if your processor is VT-capable here. If your processor is VT-capable, “Intel Virtualization Technology” will be listed as one of the supported features.

On Intel CPUs, it is expected that the BIOS (or other firmware) will either enable or disable VT-x at boot, and will then lock the setting so that it cannot be subsequently changed without powering down the CPU. Note that VT-x must be both enabled and locked before software can make use of the VMX extensions. This operation must be performed for each core of a multi-core system, and for each thread of a multi-threaded system.

Because the VT-x setting is typically locked at power on, it is necessary to fully power down the system after changing any VT-x options in the BIOS. A simple reboot is not sufficient! After saving your BIOS changes, I recommend that you actually pull the power cord(s) out of the wall and wait ten seconds. For laptop systems, you may have to remove the battery as well.

Some BIOSes do not have VT-x enable/disable options. Some BIOSes with such an option fail to enable/disable VT-x on each core of a multi-core system or on each thread of a multi-threaded system. To verify that VT-x is enabled and locked on each core/thread of your system, download the attached ISO, burn it to a CD, and boot your host from the CD.

If the ISO reports that your CPU is not VT-capable, and your CPU should be VT-capable, you may have encountered an Intel chip erratum (possibly AW67 or AV69, depending on the CPU). The effect of this erratum is that some CPU features are reported incorrectly after a particular machine specific register has been updated by the BIOS. The solution to this problem is to obtain an updated BIOS from your system vendor.

If the ISO reports that VT-x is unlocked on one or more threads/cores, a workaround is available for current VMware hosted products. Simply add the following option to your system-wide configuration file:

hv.enableIfUnlocked = TRUE

On Linux systems, the system-wide configuration file is /etc/vmware/config. On Windows systems, the system-wide configuration file varies according to VMware product and Windows version. For VMware Workstation on XP hosts, the system-wide configuration file is C:\Documents and Settings\All Users\Application Data\VMware\VMware Workstation\config.ini. For VMware Workstation on Vista hosts, the system-wide configuration file is C:\ProgramData\VMware\VMware Workstation\config.ini. For other VMware products, adjust the path appropriately.

Note that this workaround can also be helpful if your BIOS fails to enable and lock VT-x when waking from hibernation. However, the ISO is not helpful in diagnosing this failure. If VT-x works after a power-cycle but fails after resuming from hibernation, then you should try the above workaround.

If your BIOS specifically disables VT-x and locks it in that state, then it is impossible to subsequently enable VT-x through software.

Note that VT-x is unavailable to normal software if you have enabled “trusted execution,” which restricts VT-x to “trusted” code. You should disable trusted execution in the BIOS and power-cycle the system.”

VT.ISO