Bypass PAE, NX, and SSE2 upgrade requirements from Windows 8 to Windows 8.1

You can't reliably circumvent them unless you manage to at least obtain the source code for all of Windows and recompile it. (And even then I have some doubts.)

If a program requires SSE2 or something like that, it is generally because it was compiled to use SSE2 instructions – in which case bypassing the requirement checks would mean the program would crash as soon as it reached a place where those instructions were used, because your CPU didn't recognize them. (Having that happen for the OS itself wouldn't be nice, would it?).

Sometimes Windows has code for CPUs both with some feature and without, and detects the supported features at boot time – but I doubt it would bother doing that for something that was listed as required in the first place.

How to upgrade from Windows 8 64-bit to Windows 8.1 32-bit:

I found the answer but it took me some time.

1. Acquire this file: en_windows_8_1_x86_dvd_2707392.iso

   If you're not an MSDN subscriber, you can torrent it, but keep in mind sharing copyrighted content is illegal.

2. Download Virtual CloneDrive and mount the ISO file to a virtual drive.

3. Copy the entire contents of the drive to a new folder. Add the ei.cfg file to install windows without a product key.

   [EditionID]
   [Channel]
   Retail
   [VL]
   0
   

4. Use ImgBurn to make your new folder into an ISO file.

5. Create a Windows 8 bootable USB key and boot from the device.

6. Install Windows 8.1 without a product key. Select the version of Windows you bought (Windows 8 or Windows 8 Pro).

7. After installation you can activate Windows 8.1 32-bit with your legit OEM product key.

Note: This is what I had to do to install the 32-bit version since my processor does not support PrefetchW and LAHF/SAHF. When I bought Windows 8, it did not come with a 32-bit CD, hence step 1. My product key did not work until after Windows 8.1 was installed, hence step 2-4.

Windows 8.1 requires 3 new CPU features that Windows 8 doesn't require:

To install a 64-bit OS on a 64-bit PC, your processor needs to support CMPXCHG16b, PrefetchW, and LAHF/SAHF

http://windows.microsoft.com/en-gb/windows-8/system-requirements