I was able to get Windows 8.0 Pro with Media Center (64 bit) to install on my Dell XPS 400 (with 2.8GHz Pentium D processor and A07 BIOS), but I was unable to perform the Windows 8.1 upgrade. The 8.1 upgrade failed due to CPU limitations, i.e., the CPU did not support at least one of the required CPU functions, PAE, NX, SSE2 plus LAHF-SAHF, CX16 and PREFETCHW. As I understand it, the Pentium D does not support PREFETCHW, so the 8.1 upgrade is a non-starter. The only way around this issue is to go back to 32 bit Win 8 Pro.Then the upgrade to Win 8.1 Pro (32 bit) is supposed to work. I am not willing to go through all of these machinations, so I am sticking with Win 8 Pro 64 bit.