[Identifying genuine] Windows 2000

hi i just need ur some info from expert user abt original windows
how can i check that windows 2000 is original windows?..
n how can i check windows xp is original liscence windows?.
if i buy original windows 2000 how do i know it's really microsoft original windows?..

plz explain me here i already went to site n how to tell from microsoft website but not much info abt windows 2000
so plz let me explaint these things...
difference between original cd, do need activitation (windows 2000 professional) or register what is difference between them?.