Auto Fill help

I am trying to set up something along these lines in Word 2003 and am not
honestly sure of even where to start. Basically I have a word document that
has fields that need to be autofilled. The kicker is that I need to have
some sort of system where I type in an identifier number which pulls info
from and excel sheet attached to that number and that info autofills the
fields in the word doc. Is this something I can do or am I going to need an
entire different program? My boss thinks I am able to do this with the word
system we have, but Im walking in the dark right now. Any help is greatly
appreciated