How apply service pack 6.0a

I (a humble grant writer and sometime network admin. cuz we don’t have $ to pay a real one) want to apply SP 6.0a to our NT 4.0 server (SP 3 on it). What is best, safest way to do so. Our tape backup drive is dead. How can I back up everything (including NT registry) to a workstation prior to SP install? THANKS!