This was news to me…
(though I dont fully understand it and may never need to)
http://windows.microsoft.com/en-GB/windows-8/faq# from Micrsoft site FAQ
What is PAE, NX, and SSE2 and why does my PC need to support them to run Windows 8 Release Preview?
Physical Address Extension (PAE), NX processor bit (NX), and Streaming SIMD Extensions 2 (SSE2) refer to features of the processor. PAE enables 32-bit processors to access more than 4 GB of physical memory on capable versions of Windows and is a prerequisite for NX. NX allows the processor to help guard the PC from attacks by malicious software. SSE2 (a standard on processors for a long time) is an instruction set that is increasingly used by third-party applications and drivers. For Windows 8, we require that your processor support NX for security reasons to ensure that malware defense features work reliably. To enhance the reliability of third-party applications and drivers running in Windows 8, SSE2 is also required. If your PC does not support PAE, NX, and SSE2 you won’t be able to install Windows 8 Release Preview.