Installing Microsoft Exchange 5.0 or 5.5

hello again,
just wanted to hop on here and pick your tech brains. i have gotten great responses in the past using this site so hopefully luck will strike one more time.

OKAY, here is my delema i need to install “Microsoft Exchange 5.0 or 5.5 on a network. i don’t know which of the two “exchange’s” it will be, i am just wondering if any of your networking guru’s have a minute to drop a line and lend a quick hand.
thanks
k