Prefs -> Options -> checkmark: allow multiple instances
If your folders are arranged by genre, open another instance of Winamp, clear the playlist (List button -> new), add the dir of your choice (Add button -> Add dir) . . . repeat as necessary.
Or, with the initial instance of Winamp already open
Rt click the dir in Explorer -> play in Winamp
(Requires "Dir Context Menus" to be checked in Prefs -> Filetypes)
A new instance of Winamp will open with just these files in the playlist.
Or, if and when you save the playlists (List button -> save), you can open the playlists in Explorer and they'll each open in a new instance of Winamp.