Physical Address Extension (PAE), NX processor bit (NX) and Streaming SIMD Extensions 2 (SSE2) are features of the processor, and they're needed to run Windows 8.1.
PAE gives 32-bit processors the ability to use more than 4 GB of physical memory on capable versions of Windows, and is a prerequisite for NX.
NX helps your processor guard the PC from attacks by malicious software.
SSE2 is a standard instruction set on processors that is increasingly used by third-party applications and drivers.
If your PC doesn't support PAE, NX and SSE2, you won't be able to install Windows 8.1. When you download Windows 8.1 from the Store or run Upgrade Assistant, we'll let you know if your processor doesn't support PAE and SSE2, if it might not support NX or if NX is turned off in the PC BIOS. If NX appears to be off, the installer will try to turn it on during installation and, if it can't, it will return your PC to the current operating system.
You can check whether your processor supports NX and turn it on if it is turned off by going into your PC's BIOS. How you get to the BIOS varies depending on your PC manufacturer. On most PCs, you need to press a key (like F2, F12, Delete or Esc) or a key combination immediately after you turn on your PC, but before Windows starts. For details, check the info that came with your PC or your PC manufacturer's website.
Once you're in the BIOS, look for the NX or XD settings on the Security tab to turn on NX support. If the BIOS setting for NX support isn't available on your PC, you might need to contact the PC manufacturer for info on updating the BIOS. If you need help with getting to the BIOS on your PC or turning on NX, contact support.
If you've bought Windows 8.1 and can't install it, contact support.