How to create a login page in c#

I won't suggest you to store names and password in xml file,since it can be easily changed by anyone.

For WinForms, here is my solution. If you have learnt Sql Server or any other RDBMS language, then:
1.Create a table in database.
2.The table will store username and password in it.

Now create a Winform. Add 2 textboxes and a button to form. Make the form primary form. In the form properties, set,
1. StartPosition = CenterScreen.
2. ShowInTaskbar = False
3. FormBorderStyle = none

The textboxes will ask for username and password. In the event for button_click, the username and password will be checked in the table.(If you have learnt SQlServer or any other RDBMS language, you will know how to check).
Only when the user name and password match, the form asking for username and password will be closed, and new form which you want to show, will be visible.

If you are still unclear about anything, then ask again.

Probably this article will help: ASP.NET Forms Authentication - Part 1
In other words:
1. You should configure web app to use forms authentication;
2. You should check crentials and set auth token for user if all is correct;
You could use your custom logic & design or use login control. In most complex case you even could create custom membership provider.

 public static void Main()
    {
       string username, password;
       int ctr = 0;
       Console.Write("\n\nCheck username and password :\n");
	   Console.Write("N.B. : Default user name and password is :abcd and 1234\n");
       Console.Write("------------------------------------------------------\n"); 
         
        do
        {
			Console.Write("Input a username: ");
			username = Console.ReadLine();
 
			Console.Write("Input a password: ");
			password = Console.ReadLine();
			
             if(username != "abcd" || password != "1234")
             ctr++;
             else
             ctr=1;
     
        }
        while((username != "abcd" || password != "1234")  && (ctr != 3));
	 
        if (ctr == 3)
            Console.Write("\nLogin attemp three or more times. Try later!\n\n");
        else   
            Console.Write("\nThe password entered successfully!\n\n");		
    }
}