Parent-Child Domain creation in Win2003

Dear

I have 3 companies which are residing in one building and also run by one management.so we dedided one server for three companies. i want to create Parent domain as ” XYZ” and y and x for child domain. how can i create parent and child domain? i created parent doamin.when i run dcpromo command for child domain.it is asking removing active directory. pls help me to create parent and child domain setp by step.awaiting your favorite answer.