ecalling that I encountered a similar issue when trying to install Hyper-V on this computer, I Bingled for the Windows 8 RP CPU requirements and found the following in this white paper:
"No-eXecute (NX) is a processor feature that allows marking of memory pages as non-executable. The feature allows the CPU to help guard the system from attacks by malicious software. When the NX feature is enabled on a system, it prevents malicious software code from being placed in accessible regions of memory to be executed when control reaches that memory location. Windows 8 requires that systems must have processors that support NX, and NX must be turned on for important security safeguards to function effectively and avoid potential security vulnerabilities."
With that information straight from the horse's mouth, I rebooted my machine and headed to the relevant BIOS setting. For reference, I am running a Core 2 Duo e6600 on an ASUS P5W DH Deluxe. You can normally access the BIOS settings by hitting delete while your computer is starting up: