Code:

void __fastcall TCalc::FormKeyDown(TObject *Sender, WORD &Key,
      TShiftState Shift)
{
if(Key==VK_SEPARATOR){
if(Edit1->Text=="")
        Edit1->Text="0.";
else{
        if(zarez==true){
                Edit1->Text=Edit1->Text + ".";
                zarez=false;
        }
        else
                ShowMessage("Samo jedan zarez je dozvoljen.");
    }
  }
}