Opening new window or Message Box in Login using WPF MVVM.

Hi,

I am a beginner when it comes to MVVM. I am creating a simple Login page which takes user name and password. As usual if user name and password matches it should open a new window and if user name and password does not matches it should show a message box. Well i think i can accomplish it by creating object of new window and then by calling Show() method in the ViewModel class, but i don't think it is right approach using MVVM. So please suggest what i need to do so that my Login Page should follow pure MVVM.

XAML code<Window x:Class="WpfApplication1.View.LoginWin"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:viewModel="clr-namespace:WpfApplication1.ViewModel"
        Title="LoginWin" Height="300" Width="300"><Window.DataContext><viewModel:LoginViewModel></viewModel:LoginViewModel></Window.DataContext><Grid><Grid><TextBox Height="23" HorizontalAlignment="Left" Margin="96,74,0,0" Name="textBox1" VerticalAlignment="Top" Width="120" Text="{Binding Path=UserName,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay}" /><TextBox Height="23" HorizontalAlignment="Left" Margin="96,113,0,0" Name="textBox2" VerticalAlignment="Top" Width="120" Text="{Binding Path=Password,UpdateSourceTrigger=PropertyChanged}" />       </Grid><Button Content="Login" Height="23" HorizontalAlignment="Left" Margin="103,149,0,0" Name="button1" VerticalAlignment="Top" Width="75" Command="{Binding LoginCommand}" /></Grid></Window>


LoginViewModel.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Input;
using WpfApplication1.Commands;

namespace WpfApplication1.ViewModel
{
  public class LoginViewModel:ViewModelBase
    {
      private string _userName;
      private string _password;
      private ICommand _loginCommand;

      public string UserName
      {
          get { return _userName; }
          set { _userName = value; OnPropertyChanged("UserName"); }
      }

      public string Password
      {
          get { return _password; }
          set { _password = value; OnPropertyChanged("Password"); }
      }

      public ICommand LoginCommand
      {
          get
          {
              if (_loginCommand == null)
              {
                  _loginCommand = new RelayCommand(new Action<object>(CheckLogin), new Predicate<object>(CanUserLogin));
              }
              return _loginCommand;
          }
      }

      private void CheckLogin(object o)
      {
          if (UserName == "abc" && Password == "123")
          {
            //stuff to open new window
          }
         else
          {
              //stuff to show a message box for incorrect login     credentials
          }

      
      }

      private bool CanUserLogin(object sender)
      {
          return !(string.IsNullOrEmpty(UserName) || string.IsNullOrEmpty(Password)) ;
      }
    }
}

RelayCommand.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Input;

namespace WpfApplication1.Commands
{
  public class RelayCommand:ICommand
  {
      #region Class Variables
      private readonly Predicate<object> _canExecute;
      private readonly Action<object> _execute1;
      #endregion

      #region Constructor
      public RelayCommand(Action<object> execute)
          : this(execute, null)
      { 
      }

      public RelayCommand(Action<object> execute, Predicate<object> canExecute)
      {
          if (execute == null)
              throw new ArgumentNullException("execute");
          _execute1 = execute;
          _canExecute = canExecute; 
      }
      #endregion

      public bool CanExecute(object parameter)
        {
            return _canExecute == null ? true : _canExecute(parameter);
        }

      public event EventHandler CanExecuteChanged
      {
          add
          {
              if (_canExecute != null)
              {
                  CommandManager.RequerySuggested += value;
              }
          }
          remove
          {
              if (_canExecute != null)
              {
                  CommandManager.RequerySuggested -= value;
              }
          }
      }

        public void Execute(object parameter)
        {
            _execute1(parameter);     
        }
    }
}