[Страдалецъ]

И что может неполучаться, поразительно.
dfm-файл

Код:

object Form24: TForm24
  Left = 0
  Top = 0
  Caption = 'Form24'
  ClientHeight = 339
  ClientWidth = 635
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Edit1: TEdit
    Left = 32
    Top = 32
    Width = 121
    Height = 21
    TabOrder = 0
    Text = 'Edit1'
    OnKeyPress = Edit1KeyPress
  end
end

Код:

unit Unit24;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm24 = class(TForm)
    Edit1: TEdit;
    procedure Edit1KeyPress(Sender: TObject; var Key: Char);
  end;

var
  Form24: TForm24;

implementation

{$R *.dfm}

procedure TForm24.Edit1KeyPress(Sender: TObject; var Key: Char);
begin
if not (Key in ['0'..'9', #8]) then Key := #0;
end;

end.