uses
 CommCtrl;
procedure SetNodeBoldState(Node: TTreeNode; Value: Boolean);
var
 TVItem: TTVItem;
begin
 if not Assigned(Node) then Exit;
 with TVItem do
 begin
  mask := TVIF_STATE or TVIF_HANDLE;
  hItem := Node.ItemId;
  stateMask := TVIS_BOLD;
  if Value then state := TVIS_BOLD
  else
  state := 0;
  TreeView_SetItem(Node.Handle, TVItem);
 end;
end;
// Example: Make the first node bold.
// Beispiel: Erster Eintrag fett machen.
procedure TForm1.Button1Click(Sender: TObject);
begin
 SetNodeBoldState(TreeView1.Items[0], True);
end;

uses CommCtrl;
...
procedure SetNodeState(node :TTreeNode; Flags: Integer);
var
 tvi: TTVItem;
begin
 FillChar(tvi, Sizeof(tvi), 0);
 tvi.hItem := node.ItemID;
 tvi.mask := TVIF_STATE;
 tvi.stateMask := TVIS_BOLD or TVIS_CUT;
 tvi.state := Flags;
 TreeView_SetItem(node.Handle, tvi);
end;
//И вызываем:
SetNodeState(TreeView1.Selected, TVIS_BOLD); // Текст жиpным
SetNodeState(TreeView1.Selected, TVIS_CUT); // Иконкy бледной (Ctrl+X)
SetNodeState(TreeView1.Selected, TVIS_BOLD or TVIS_CUT); // Текст жиpным
SetNodeState(TreeView1.Selected, 0); // Ни того, ни дpyгого