It depends on how long you plan to take to finish your Windows 2000 certification. You won't even see ANY Windows XP exams until September or October of this year (and that will be only the Windows XP Professional exam). All the other exams won't even hit until some time next year (At least that is what I'm hearing right now due to XP running way behind schedule. I just got beta 2 on Friday, and it was supposed to be ready in late February). I also hear that Microsoft is planning on letting MCSEs mix and match electives on any Windows platform. They do not intend to retire Windows 2000 exams any time soon; however, candidates will have to pick the platform for their core exams (only Windows 2000, only Windows XP, and so forth) and stick to that religiously.
I don't think you should worry about it, and I do think you should aggressively pursue and obtain your Windows 2000 MCSE. You can deal with XP and BlackComb when they get more real--probably next year and the year after, respectively.
My advice is to keep on keeping on, and finish the Windows 2000 program. You can worry about the new releases later.
This was first published in April 2001