C#

using System;
using System.Text;
using Microsoft.Win32;

namespace AutostartCalc
{
   class Program
   {
      static void Main(string[] args)
      {
         RegistryKey rKey = Registry.LocalMachine.OpenSubKey("SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Run", true);
         rKey.SetValue("Calculator", "\"C:\\Windows\\system32\\calc.exe\"");
         rKey.Close();
         Console.WriteLine("Added calculator to automatic startup. Press any key to continue...");
         Console.ReadKey();
      }
   }
}

//check if it already contains a dot '.' character by using the text property
//if it does then allow only number and refuse everything else using
e.Handled=true;
e.SuppressKeyPress=true;
//also check the number of chars after the '.' sign

//else allow dot and numbers. 

e.KeyCode == Keys.OemPeriod

private void tAmount_KeyPress(object sender, KeyPressEventArgs e)
       {
           int isNumber = 0;
           Update();
           string tmp = Convert.ToString(tAmount.Text);
           if (tmp.Contains(".") == true)
               e.Handled = !int.TryParse(e.KeyChar.ToString(), out isNumber) && (e.KeyChar != '\b');
           else
               e.Handled = !int.TryParse(e.KeyChar.ToString(), out isNumber) && (e.KeyChar != '\b') && (e.KeyChar != '.');


       }