.NET Active Directory Wrapper    
Access Active Directory From Your C# or VB .Net Code
 

Active Directory
Introduction
Objects
Object Properties

DirectoryServices Namespace
Introduction
DirectoryEntry Class

How to .. in C# .NET
Create User
Disable User
Enable User
Authenticate a User
Change User Password
Get DirectoryEntry of a user
Get Users in a Group
Get Object Property

Other Resources
References

Getting the DirectoryEntry Object from Active Directory for a given username

public static DirectoryEntry GetUser(string UserName)
{
//create an instance of the DirectoryEntry
DirectoryEntry de = GetDirectoryObject("/" + GetLDAPDomain());

//create instance fo the direcory searcher
DirectorySearcher deSearch = new DirectorySearcher(de);

deSearch.SearchRoot =de;
//set the search filter
deSearch.Filter = "(&(objectCategory=user)(cn=" + UserName + "))";
//deSearch.SearchScope = SearchScope.Subtree;

//find the first instance
SearchResult results= deSearch.FindOne();

//if found then return, otherwise return Null
if(results !=null)
{
//de= new DirectoryEntry(results.Path,ADAdminUser,ADAdminPassword,AuthenticationTypes.Secure);
//if so then return the DirectoryEntry object
return results.GetDirectoryEntry();
}
else
{
return null;
}
}