If I recall correctly, Microsoft's line is that "CURRENT" software/OS will be available to be patched--that means anything in the XP Family that's NOT SP2 (Service Pack 2) may not get a patch.
I don't think Server2000 will get any free patches either--in fact, you CAN buy the patches for non-supported SW, for about $4K a pop.
Media Center is XP Professional with add ons, so make sure it's up to date as well. Windows update "Can" be your friend!