I'm not precisely sure why it doesn't work, but 2000 is basically "NT 5.0". Back when it was released, Microsoft still had two distinct lines of Windows, the Win95 (aka glorified DOS) kernel, and the NT kernel. These two lines were merged into one with the release of Windows XP, which has also been called "NT 5.1". XP is still pretty close to 2000, but XP also introduced a fair number of new features. I would guess that Autocad now uses some of these new features, but I'm not sure exactly which ones are responsible for making it incompatible with Windows 2000.