To compile the source on Win XP download this dependency pack I put together: http://www.mediafire.com/file/ozha9c7kmhg8ik3/UltraEdDeps.zip. It contains SP3 for WinXP, SP6 for VS 6.0 and DirectX 8.1 SDK.
The reason why UltraEd only runs on Windows XP/NT is due to the ROM build process. I rely upon the N64 SDK to do the final ROM construction. I realize I could still build an editor for a modern OS and then only build the ROM on XP/NT but I wanted the editor to feel sort of like Unity 3D. Everything within one environment! :)