.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

How to change a user's Active Directory Password

public static void SetUserPassword (string UserName, string NewPassword)
{
//get reference to user
string LDAPDomain="/CN="+ UserName+",CN=Users," + GetLDAPDomain() ;
DirectoryEntry oUser= GetDirectoryObject(LDAPDomain);//,UserName,OldPassword);
oUser.Invoke("SetPassword",new Object[]{NewPassword});
oUser.Close();
}