back-up server

i`m experimenting with setting up a back up server. i want to have another server working as a back up to main server just in case main server crashed the other server keeps all data and computers running till the main server is fixed.
i have windows 2000 server on both computer , and installed active directory and dns on both computer , created two forests. my question is what steps i should take to get back-up server to work i have tried everything and everytime i shut main server down i can`t connect to back-up server to resume normal work.
thanks for any suggestions
p.s. do i need to have identical softwares in both computers?