const
TVS_CHECKBOXES = $00000100;

PRocedure SetComCtrlStyle(WinCtrl: TWinControl; Value: Integer; UseStyle: Boolean);
var
Style: Integer;
begin
  if WinCtrl.HandleAllocated then
  begin
    Style := GetWindowLong(WinCtrl.Handle, GWL_STYLE);
    if not UseStyle then
    Style := Style and not Value
    else Style := Style or Value;
    SetWindowLong(WinCtrl.Handle, GWL_STYLE, Style);
  end;
end;

SetComCtrlStyle(TreeView1, TVS_CHECKBOXES, True);

SetWindowLong(TreeView1.Handle, GWL_STYLE, GetWindowLong(TreeView1.Handle, GWL_STYLE) or $00000100);