Microsoft confirms some older AMD processors do not support Windows 8.1
 

Short story: AMD's early CPUs, i.e. Athlon x2 64, Opteron 185 lack CMPXCHG16b instructions. Until Windows 8 (apparently) MS implemented a software solution into the OS to support this instruction (so I read). Windows 8.1 refuses to install on machines based on those CPUs. Windows 8 works great.

http://www.neowin.net/news/microsoft...ort-windows-81