For 64 bit installations of Windows 8.1 the CPU must support CMPXCHG16b, PrefetchW and LAHF/SAHF instructions. PrefetchW is an AMD 3DNow! instruction and Intel desktop processors became compatible from family 15/model 6/stepping 1 (Cedar Mill, the 65 nM final revision of the Pentium 4 released on January 5, 2006) thus I should think all 90 nM Pentium 4 era CPUs and earlier are incompatible (Prescott and earlier).