Win32: How to validate credentials against Active Directory?

For the native equivalnt of your valid .NET solution see this MSDN page and ldap_bind

Howerver I think that LogonUser is the right API for the task when use with LOGON32_LOGON_NETWORK. Note that the limitation of SE_CHANGE_NOTIFY_NAME is only for Windows 2000 (so Windows XP and newer do not require this priviledge) and that by default SE_CHANGE_NOTIFY_NAME is enabled for all users. Also the MSDN page says

The SE_TCB_NAME privilege is not required for this function unless you are logging onto a Passport account.

In this case you are logging onto an AD account so SE_TCB_NAME is not required.

I might as well post the native code to validate a set of Windows credentials. It took a while to implement.

function TSSPLogon.LogonUser(username, password, domain: string; packageName: string='Negotiate'): HRESULT;
var
    ss: SECURITY_STATUS;
    packageInfo: PSecPkgInfoA;
    cbMaxToken: DWORD;
    clientBuf: PByte;
    serverBuf: PByte;
    authIdentity: SEC_WINNT_AUTH_IDENTITY;
    cbOut, cbIn: DWORD;
    asClient: AUTH_SEQ;
    asServer: AUTH_SEQ;
    Done: boolean;
begin
{
    If domain is blank will use the current domain.
    To force validation against the local database use domain "."

    sspiProviderName is the same of the Security Support Provider Package to use. Some possible choices are:
            - Negotiate (Preferred)
                        Introduced in Windows 2000 (secur32.dll)
                        Selects Kerberos and if not available, NTLM protocol.
                        Negotiate SSP provides single sign-on capability called as Integrated Windows Authentication.
                        On Windows 7 and later, NEGOExts is introduced which negotiates the use of installed
                        custom SSPs which are supported on the client and server for authentication.
            - Kerberos
                        Introduced in Windows 2000 and updated in Windows Vista to support AES) (secur32.dll)
                        Preferred for mutual client-server domain authentication in Windows 2000 and later.
            - NTLM
                        Introduced in Windows NT 3.51 (Msv1_0.dll)
                        Provides NTLM challenge/response authentication for client-server domains prior to
                        Windows 2000 and for non-domain authentication (SMB/CIFS)
            - Digest
                        Introduced in Windows XP (wdigest.dll)
                        Provides challenge/response based HTTP and SASL authentication between Windows and non-Windows systems where Kerberos is not available
            - CredSSP
                        Introduced in Windows Vista and available on Windows XP SP3 (credssp.dll)
                        Provides SSO and Network Level Authentication for Remote Desktop Services
            - Schannel
                        Introduced in Windows 2000 and updated in Windows Vista to support stronger AES encryption and ECC (schannel.dll)
                        Microsoft's implementation of TLS/SSL
                        Public key cryptography SSP that provides encryption and secure communication for
                        authenticating clients and servers over the internet. Updated in Windows 7 to support TLS 1.2.

    If returns false, you can call GetLastError to get the reason for the failure
}


    // Get the maximum authentication token size for this package
    ss := sspi.QuerySecurityPackageInfoA(PAnsiChar(packageName), packageInfo);
    if ss <> SEC_E_OK then
    begin
        RaiseWin32Error('QuerySecurityPackageInfo "'+PackageName+'" failed', ss);
        Result := ss;
        Exit;
    end;

    try
        cbMaxToken := packageInfo.cbMaxToken;
    finally
        FreeContextBuffer(packageInfo);
    end;

    // Initialize authorization identity structure
    ZeroMemory(@authIdentity, SizeOf(authIdentity));
    if Length(domain) > 0 then
    begin
        authIdentity.Domain := PChar(Domain);
        authIdentity.DomainLength := Length(domain);
    end;

    if Length(userName) > 0 then
    begin
        authIdentity.User := PChar(UserName);
        authIdentity.UserLength := Length(UserName);
    end;

    if Length(Password) > 0 then
    begin
        authIdentity.Password := PChar(Password);
        authIdentity.PasswordLength := Length(Password);
    end;

    AuthIdentity.Flags := SEC_WINNT_AUTH_IDENTITY_ANSI; //SEC_WINNT_AUTH_IDENTITY_UNICODE

    ZeroMemory(@asClient, SizeOf(asClient));
    ZeroMemory(@asServer, SizeOf(asServer));

    //Allocate buffers for client and server messages
    GetMem(clientBuf, cbMaxToken);
    GetMem(serverBuf, cbMaxToken);
    try
        done := False;
        try
            // Prepare client message (negotiate)
            cbOut := cbMaxToken;
            ss := Self.GenClientContext(@asClient, authIdentity, packageName, nil, 0, clientBuf, cbOut, done);
            if ss < 0 then
            begin
                RaiseWin32Error('Error generating client context for negotiate', ss);
                Result := ss;
                Exit;
            end;

            // Prepare server message (challenge).
            cbIn := cbOut;
            cbOut := cbMaxToken;
            ss := Self.GenServerContext(@asServer, packageName, clientBuf, cbIn, serverBuf, cbOut, done);
            if ss < 0 then
            begin
                {
                    Most likely failure: AcceptServerContext fails with SEC_E_LOGON_DENIED in the case of bad username or password.
                    Unexpected Result:   Logon will succeed if you pass in a bad username and the guest account is enabled in the specified domain.
                }
                RaiseWin32Error('Error generating server message for challenge', ss);
                Result := ss;
                Exit;
            end;

            // Prepare client message (authenticate).
            cbIn := cbOut;
            cbOut := cbMaxToken;
            ss := Self.GenClientContext(@asClient, authIdentity, packageName, serverBuf, cbIn, clientBuf, cbOut, done);
            if ss < 0 then
            begin
                RaiseWin32Error('Error generating client client for authenticate', ss);
                Result := ss;
                Exit;
            end;

            // Prepare server message (authentication).
            cbIn := cbOut;
            cbOut := cbMaxToken;
            ss := Self.GenServerContext(@asServer, packageName, clientBuf, cbIn, serverBuf, cbOut, done);
            if ss < 0 then
            begin
                RaiseWin32Error('Error generating server message for authentication', ss);
                Result := ss;
                Exit;
            end;
        finally
            //Free resources in client message
            if asClient.fHaveCtxtHandle then
                sspi.DeleteSecurityContext(@asClient.hctxt);

            if asClient.fHaveCredHandle then
                sspi.FreeCredentialHandle(@asClient.hcred);

            //Free resources in server message
            if asServer.fHaveCtxtHandle then
                sspi.DeleteSecurityContext(@asServer.hctxt);

            if asServer.fHaveCredHandle then
                sspi.FreeCredentialHandle(@asServer.hcred);
        end;
    finally
        FreeMem(clientBuf);
        FreeMem(serverBuf);
    end;

    Result := S_OK;
end;

Note: Any code released into public domain. No attribution required.

There is a win32 API function called ldap_bind_s. The ldap_bind_s function authenticates a client against LDAP. See MSDN documentation for more information.