bool isLegalChar = false;			//class-level variable


//Use both the Keypress & KeyDown events to trap your keys. Keydown fires first!!!
//I allow a-z, space bar, & backspace. No numbers or special keys!

//Define the legal keys in KeyDown and let Keypress do the actual filtering!

        private void txtCustomer_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (isLegalChar)
                e.Handled = false;
            else 
                e.Handled = true;
            
        }

        private void txtCustomer_KeyDown(object sender, KeyEventArgs e)
        {
            isLegalChar = false;
            if ((e.KeyCode >= Keys.A && e.KeyCode <= Keys.Z) | (e.KeyCode == Keys.Space | e.KeyCode == Keys.Back))
                isLegalChar = true;
            else
                isLegalChar = false;
        }
        
        
        //Another way to do it
        
        
        // Boolean flag used to determine when a character other than a number is entered.
private bool nonNumberEntered = false;

// Handle the KeyDown event to determine the type of character entered into the control.
private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
{
    // Initialize the flag to false.
    nonNumberEntered = false;

    // Determine whether the keystroke is a number from the top of the keyboard.
    if (e.KeyCode < Keys.D0 || e.KeyCode > Keys.D9)
    {
        // Determine whether the keystroke is a number from the keypad.
        if (e.KeyCode < Keys.NumPad0 || e.KeyCode > Keys.NumPad9)
        {
            // Determine whether the keystroke is a backspace.
            if(e.KeyCode != Keys.Back)
            {
                // A non-numerical keystroke was pressed.
                // Set the flag to true and evaluate in KeyPress event.
                nonNumberEntered = true;
            }
        }
    }
}

// This event occurs after the KeyDown event and can be used to prevent
// characters from entering the control.
private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
{
    // Check for the flag being set in the KeyDown event.
    if (nonNumberEntered == true)
    {
        // Stop the character from being entered into the control since it is non-numerical.
        e.Handled = true;
    }
}