Email Boxes

We have just one server that serves as our File/Proxy/Mail serve. I have uninstall MSExchange 5.5 from our server because of some problem. When I re-install it, I can not find the mail box option at the User Manager for Domain. Can you please advisewhat is the correct sequence & procedure of install Exchange in relation w/NT4 Server, Proxy, & IIS and what should I do to rectify this problem.