The first limitation introduced in Windows 8 is the Physical Address Extension (PAE) requirement.
While you can install Windows XP, Vista and 7 on the original Intel Pentium for testing purposes, the PAE was implemented first in the Intel Pentium Pro in 1995. Therefore this first limitation make the older Pentium too obsolete to install Windows 8.
The second limitation introduced is the Streaming SIMD Extensions 2 (SSE2) requirement.
It was introduced with the Intel Pentium 4 in late 2000. This limitation make the Intel Pentium II, III and their AMD competitors of the late 90s and early 2000s too obsolete to install Windows 8.
The third limitation is the Never eXecute (NX bit) requirement.
It was introduced around 2003, but since at that time many processors were sold without this technology, many Pentium 4 and Athlon XP are too obsolete to install Windows 8.
At the software level, it was implemented from Windows XP SP2 and Windows Server 2003 SP1.
Therefore, Windows 8 can be installed if you have a processor from about 2004 or newer.
This post has been edited by Agorima: 24 August 2012 - 05:13 PM