OWA EXCH2003 – HTTP/1.1 503 service message unavailable

I have 2 nodes back-end exchange cluster environment with one OWA front-end exchange server. I am using the OWA box for OWA client authentication. I have installed all the necessary security updates along with re-installing IIS, and exchange 2003 OS. After entering username, and password on the login and I launched, I get the following error message: OWA EXCH2003 – HTTP/1.1 503 service message unavailable. Kindly tell me what to do in order to fix this problem.