Even after AMD was dropped for processors, their AMD-760 chipset was kept around for the final hardware design. nVidia provided the graphics chipset for all the machines. Due to Intel offering a lower price (a deal) for processors, subsequent SDK machines came with Pentium III's. The earliest of the SDK's, which seem to have never appeared up for auction, were AMD-based systems. These kits were basic PC's with a specific target hardware feature-set. The OS was designed from roughly 1999 through 2001, with Alpha SDK kits going out early on in 2000. That isn't to say that those services weren't modified for a low-memory environment, though. A large amount of core services were ripped from the NT source and given a home within the Xbox OS. During my own research, I compared the source tree directories and files stored within to the Windows Research Kernel, and found that, for the most part, NT and XboxNT are very close cousins. There has been lots of speculation as to whether or not the OS was completely written from scratch, or if it was simply a modified form of the NT kernel. The underlying Kernel was designed specifically for the Xbox. Those are not things that I will talk about in this post, as they are secondary to the main idea behind this thread.
Note: There are security systems in place that have been both defeated and bypassed due to failure to read Intel documentation on Microsoft's part. The entire kernel exists within the first 1MB of system RAM once it's fully booted. Internally, the OS consists of the bare-minimum boot code to transfer from x86 POST up through switching to protected mode and 32-bit kernel mode. It was designed around a heavily customized Windows Embedded line of Operating Systems. The Original Xbox Console's Operating System, herein called xboxkrnl, was based on Windows 2000 code. Disclaimer: I will not provide links to sources, as per forum rules.