iis problems on windows 2003 server standard edition

I never got this problems before until i change the windows from windows 2000 to windows 2003....
my problems is i have one application that assign as iis default web site,
this application can be access from everybody inside the intranet network.
When user started to access the application by using the server ip or server name,
the popup password display. i understand that the popup password require for user authentication.
i already did all solution for the authentication but i still have the same problem.
i even reformat the server and started all the things form A - Z but still same problems appears.
The funny things also happend to me, after i finish the windows installation and done everything
including the anonymous authentication, the application can be access without any problems but
after 30 minutes the problems start and this problems also appears on the server itself,
i means the server itself also cannot run the application.
please, everybody, i need help.. i really appreciate your help..

Have you double checked that anonymous authentication is enabled and the IUSR_computername account has the correct permissions on the website folder?

Yes, i already checked everything. That's what makes me crazy right now but thanks for reply.

Are you using an server-side programming language such as ASP or PHP?