OWA access error

I’m running MSExchange 5.5 on a Win 2k server with IIS 5.0 installed. If the server has been re-started OWA can be accessed one time. But the next time someone tries to access it, the Logon screen comes up, you can put in the login info and it seems to be trying to find the server and eventually reports back the page cannot be displayed.
I really need to upgrade Exchange, but I am hesitant to do so with the server having errors. Any ideas?