If you get this error using VMWare ESX 4.0 it is because either your CPU does not meet the minimum hardware requirements for 64bit guest operating systems or you have not properly configured the host hardware.
The following knowledge base article describes the settings required to be enabled in the bios for both AMD and Intel CPU architectures.
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1901