procedure TForm1.FormCreate(Sender: TObject);
{©Drkb v.3(2007): www.drkb.ru}
begin
 SetWindowRgn(
 ComboBox1.Handle,
 CreateRectRgn(2, 2, ComboBox1.Width - 2, ComboBox1.Height - 2), True);
end;

unit Unit1;
{©Drkb v.3(2007): www.drkb.ru}
interface
uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs, StdCtrls;
type
 TForm1 = class(TForm)
  ComboBox1: TComboBox;
  procedure FormCreate(Sender: TObject);
 private
  { Private declarations }
 public
  { Public declarations }
 end;
var
 Form1: TForm1;
implementation
{$R *.dfm}

function NewComboProc(wnd:HWND; uMsg:UINT; wParam:WPARAM; lParam:LPARAM):integer; stdcall;
 var
  r,r1:TRect;
 begin
  case uMsg of
  WM_PAINT: begin
  GetWindowRect(wnd,r);
  SetWindowRgn(wnd,CreateRectRgn(2, 2, r.Right-r.Left - 2, r.Bottom-r. - 2), True);
  end;
  WM_CTLCOLORLISTBOX:
  begin
  GetWindowRect(wnd,r);
  GetWindowRect(lParam,r1);
  if (r.Left=r1.Left) and (r.Right=r1.Right) then
  begin
  InflateRect(r1,-2,0);
  SetWindowPos(lParam,0,r.Left+2,r.Bottom-2,r1.Right-r1.Left,r1.Bottom-r1.,SWP_NOZORDER);
  end
  end;
  end;
  result:=CallWindowProc(Pointer(GetWindowLong(wnd,GWL_USERDATA)),wnd,uMsg,wParam,lParam);
 end;
procedure TForm1.FormCreate(Sender: TObject);
begin
 SetWindowLong(ComboBox1.Handle,GWL_USERDATA,SetWindowLong(ComboBox1.Handle, GWL_WNDPROC, LPARAM(@NewComboProc)))
end;
end.