In the upcoming weeks, the Bugzilla Project will be releasing new versions of Bugzilla on all branches–3.2.10, 3.4.10, 3.6.4, and 4.0rc2. Due to some important changes that are coming in those releases, you will no longer be able to install Bugzilla on Windows 2000 or Windows 2000 Server. You will, however, still be able to use a web browser on Windows 2000 or Windows 2000 Server as a client to access Bugzilla.
Note that Windows 2000 itself reached end-of-life from Microsoft in July of this year, so this is in line with that, as well.
All newer versions of Windows will still be supported. Only Windows 2000 and Windows 2000 Server support is being deprecated.