Well, WinAmp seems to use CreateWindowExW to create the window, but I can't find where the window class is registered, it doesn't seem to appear in a +class trace anywhere. The string name is "#32770", so maybe it's internal.
Indeed, here is a list of system classes:
Class Description ---------- -------------------------------------------------------------- ComboLBox The class for the list box contained in a combo box. DDEMLEvent Windows NT/Windows 2000/Windows XP: The class for Dynamic Data Exchange Management Library (DDEML) events. Message Windows 2000/Windows XP: The class for a message-only window. #32768 The class for a menu. #32769 The class for the desktop window. #32770 The class for a dialog box. #32771 The class for the task switch window. #32772 Windows NT/Windows 2000/Windows XP: The class for icon titles.