If you deployed your asp.net web site on IIS under newly installed windows 2003 , please make sure you have enabled the ASP.NET to run on IIS as it's disabled by default in Windows 2003.
To Enable ASP.NET :
1- Open IIS Management Console.
2- Open Web Service Extension .
3- Select the installed ASP.NET version (1.1 or 2.0 )
4- Click Allow from left navigation panel.