Hello!
The post might be old, but I will post anyway to help anyone with a Pentium 4 or older that wants to try 8 or the new Windows 10.
The Windows 8 RP (Release preview), the RTM (final) and the 8.1 version all have a check for PAE/NX compatibility. The same is true for the Windows 10 Technical Preview, and will probably be for Windows 10 RTM when it will be out in 2015. PAE and NX-bit must be supported in both the CPU and the motherboard to install these versions without any hacking. A typical Pentium 4 Northwood or Prescott doesn't meet these requirements (there are rare exceptions), so the only way to install Windows 8 or 8.1 or 10 there is by hacking some specific system files as described here:
[Q] Win 8, bypass PAE/NX/SSE2 check - Page 12
(I hope the link still works).
Almost forgot: Using Windows Update will probably alter these system files and you won't be able to load Windows 8 (and later) again! Don't worry, do all the updates and if you get this situation, simply boot from a Windows 7 DVD (or otherwise) and re-patch the files! It worked for me!
I did it successfully a couple of times. I could also upgrade from Windows 7 to 8 but this is WAY to tricky for the average user to do. You have to use the patches described in the article to patch the files at the appropriate folder, don't remember all the details now, but it can be done. For not-so-experienced users I recommend a backup and a clean installation (format).
Assuming we patch the relevant system files, the minimum system requirements for a clean installations match those for Windows 7, that is at least 450-500 Mhz Pentium III or equivalent AMD CPU, at least 256MB RAM (for upgrade make that 512MB) and at least 15-20GB free disk space.
Important note about graphics: If your VGA lacks WDDM drivers, the Microsoft Basic Display Adapter driver will be used, and you won't be able to use any 3D acceleration (forget games and nice screensavers). In that case, I recommend installing Windows 7 32-bit instead. In Windows 7 32-bit one can manually install Windows XP graphics drivers (XDDM drivers). You won't have aero, but you get all the acceleration your card supports. In Windows 8 and later, XDDM drivers are not supported and you are stuck with Microsoft Basic Display Adapter! So make sure you can find at least Vista 32-bit drivers for your VGA before you try Windows 8! If you cannot find Windows 8 drivers, don't worry, Windows Vista or 7 drivers work in 8 and in 10! Also some older games require some files from DirectX 9 to work properly, so I recommend installing DirectX 9 after you have installed all drivers.
About performance: Windows 8 (and probably 10) have the same performance as 7 (assuming we have WDDM graphics drivers) in the same hardware. Windows 7 are slightly slower than Windows XP at the same hardware. Windows Vista are A LOT slower than XP or even 7 and 8 in the same hardware, so avoid Vista by all means! In the worst case, go for 7 or keep XP.
