LDAP Authentication fails on Web Application

Sorry for cross posting the query. But I need a resolution as early as
possible.

I have developed an application to authenticate the user based on LDAP
Search and authentication.

The Windows application that was developed using C# works fine. But it
fails when I convert this as a DLL or Web Service. I need to host the
application on a server to use the result across different applications
and on different clients.

The code looks likes this,

System.DirectoryServices.DirectoryEntry entry;
System.DirectoryServices.DirectorySearcher mySearcher;

bool success = false;
string message = "";

try
{
entry = new System.DirectoryServices.DirectoryEntry(ldap_host,
application_user, application_password,
System.DirectoryServices.AuthenticationTypes.SecureSocketsLayer);
}

catch (Exception ex)
{
// Throws "The server is not operational error"
// Also the entry.AdsObject returns as
<error: an exception of type:
{System.Runtime.InteropServices.COMException} occurred>
}


But the server is in operations, and the code works fine with the
Desktop /Windows application.

Is there any problem with the ASPNET account? or should i really need
to do anything to make it work.

TIA