Close Window from ViewModel

Im creating a Login using a window control to allow a user to login into a WPF application that I am creating.

So far, I have created a method that checks whether the user has entered in the correct credentials for the username and password in a textbox on the login screen, binding two properties.

I have achieved this by creating a bool method, like so;

public bool CheckLogin()
{
    var user = context.Users.Where(i => i.Username == this.Username).SingleOrDefault();

    if (user == null)
    {
        MessageBox.Show("Unable to Login, incorrect credentials.");
        return false;
    }
    else if (this.Username == user.Username || this.Password.ToString() == user.Password)
    {
        MessageBox.Show("Welcome " + user.Username + ", you have successfully logged in.");

        return true;
    }
    else
    {
        MessageBox.Show("Unable to Login, incorrect credentials.");
        return false;
    }
}

public ICommand ShowLoginCommand
{
    get
    {
        if (this.showLoginCommand == null)
        {
            this.showLoginCommand = new RelayCommand(this.LoginExecute, null);
        }
        return this.showLoginCommand;
    }
}

private void LoginExecute()
{
    this.CheckLogin();
} 

I also have a command that I bind to my button within the xaml like so;

<Button Name="btnLogin" IsDefault="True" Content="Login"
        Command="{Binding ShowLoginCommand}" />

When I enter in the username and password it executes the appropriated code, whether it being right, or wrong. But how can I close this window from the ViewModel when both username and password are correct?

I have previously tried using a