function GetCurrUserName: string;
var
  Size : DWORD;
begin
  Size := MAX_COMPUTERNAME_LENGTH + 1;
  SetLength(Result, Size);
  if GetUserName(PChar(Result), Size) then
    SetLength(Result, Size)
  else
    Result := '';
end;

function ComputerNameStr: String;
var
  Size: DWORD;
begin
  Size := MAX_COMPUTERNAME_LENGTH + 1;
  SetLength(Result, Size);
  if GetComputerName(PChar(Result), Size) then
    SetLength(Result, Size)
  else
    Result := '';
end;

function Impersonate(const User, PW: string): Boolean;
var
  LogonType : Integer;
  LogonProvider : Integer;
  strAdminUser : string;
  strAdminDomain : string;
  strAdminPassword : string;
begin
  LogonType := LOGON32_LOGON_INTERACTIVE;
  LogonProvider := LOGON32_PROVIDER_DEFAULT;
  strAdminUser := USER;
  strAdminDomain := ComputerNameStr;
  strAdminPassword := PW;
  Result := LogonUser(PChar(strAdminUser), PChar(strAdminDomain), PChar(strAdminPassword), LogonType, LogonProvider, TokenHandle);
  if Result then
  begin
    Result := ImpersonateLoggedOnUser(TokenHandle);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
Var unstr, pwstr : String;
begin
     // Starte Defragmentierung
     unstr := Username.Text;
     pwstr := Password.Text;

     if Impersonate(unstr, pwstr) then
     begin
          RevertToSelf;
          if ShellExecute( Application.Handle , 'open' , PChar('dfrg.msc'), Nil, Nil, SW_NORMAL) <= 32 then
             ShowMessage('Es ist ein Fehler aufgetreten');
     end
     else
         MessageBox(0, PChar(SysErrorMessage(GetLastError)), '', 0);

end;