C++ spam bot

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace Spam_Bot_v2
{
    public partial class Form1 : Form
    {
        int amount;
        int counter;
        int speed;
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text == "")
            {
                //error please enter some text
                MessageBox.Show("Please enter text to spam");
                return;
            }

            if (textBox2.Text == "")
            {
                //error please enter some limit
                MessageBox.Show("Please enter a limit");
                return;
            }

            string spam_text_limit = textBox2.Text;
            for (int i = 0; i < spam_text_limit.Length; i++)
            {
                if (!char.IsNumber(spam_text_limit[i]))
                {
                    //error not a valid number
                    MessageBox.Show("Please enter a valid number");
                    textBox2.Text = "";
                    return;
                }
            }
            progressBar1.Value=0;
            amount = Convert.ToInt32(textBox2.Text);
            Clipboard.SetText(textBox1.Text);
            speed = trackBar1.Value;
            if (speed == 0)
                speed = 10;
            else
                speed = 11 - speed;

            System.Threading.Thread.Sleep(2500);
            progressBar1.Maximum = amount;
            counter = 0;
            timer1.Enabled = true;
        }

        private void the360GamersToolStripMenuItem_Click(object sender, EventArgs e)
        {
            System.Diagnostics.Process.Start("http://www.the360gamers.com");
        }

        private void exitToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            timer1.Enabled = false;
            counter = 0;
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            for (int i = 0; i < speed; i++)
            {
                SendKeys.Send("^{V}");
                SendKeys.Send("{ENTER}");
                counter++;
                progressBar1.Value++;
                if (trackBar1.Value == 10)
                {
                    System.Threading.Thread.Sleep(500);
                }
                if (counter >= amount)
                {
                    counter = 0;
                    timer1.Enabled = false;
                    break;
                }
            }

        }

        private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Created by Nity\nThe soul purpose of this program is to annoy people :D");
        }
    }
}