File: Microsoft Xbox (1.00.4400) [April 2002] [src].zip
This is the Barnabas Collins source code leak. It contains just the kernel.
This source code tree is known to compile the kernel/BIOS only, if the "Windows Server 2003 Driver Development Kit" (DDK) is added in its build path.

File: CPXXUPD.ZIP
Team Complex's patch (Xbox Source Tree Update v1.2)

File: xbox trunk.rar
Full Xbox Source Tree