Nagios startup page error


I have done all the necessary configuration and when i ran the pre-flight test i got no errors.
But when i accessed nagios from my browser: localhost/nagios all i get to see is the welcome page. Whenever i try to access the Tactical overview or any of the other links, i get the following error:
Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

So when i access the error log this is what i find:
Premature end of script headers: tac.cgi, referer: localhost/nagios/side.html

How do i solve the problem?

#2 … ?