I used to be into Windoze certs. But off hand, I would recommend XP based on the logic that it will be "around" longer than 2k since it came out later.
Also, it is the merging of the home OS and the business OS therefore your cert will be more applicable to more machines allowing you to be competitive in both business environments and home.
HTH!
If you don't have any certs at all, I would strongly encourage you to get COMPTIA A+ and Network+ as an introduction into the concepts and background you can use to further your education more than without taking them.
Professionally, no one is more annoying to me is a newbie MCSE that doesn't know how to fix a PC or Server internally or doesn't know the most basic concepts of networking - not M$'s idea of networking by the way!
