It is a stretch to call it just a PC built from COTS hardware. The NV2A GPU was a modified version on the NV20 and included the Northbridge. The MCPX Southbridge was custom. The DVD was customized for anti-piracy. The CPU is a (slightly) customized PIII. If it was all COTS, the XBox XEmu emulator would be further along than it is now.
Didn’t the SouthBridge turn up in some Nvidia ATX boards ? I recall positively salivating on realising that one particular Nvidia motherboard gave us a hardware encoded Dolby digital signal, just like the Xbox..
https://classic.copetti.org/writings/consoles/xbox/