It is impossible to install Windows 8 or Windows Server 2012 inside virtual machine. Either of two errors is observed:
NX support is not enabled in host CPU.
Use CPU which has NX support. For more information refer to the following Microsoft Knowledge Base article: PAE/NX/SSE2 Support Requirement Guide for Windows 8
If the CPU has the execution prevention mechanism (NX or XD) support, then it is necessary to enable the feature in BIOS. Refer to the hardware vendor documentation for more details on how to enable "No execute" feature.