Winamp maintains two copies of the active playlist editor's list. They are called "winamp.m3u" and "winamp.m3u8". They are in the "C:\Users\***\AppData\Roaming\Winamp" folder (replace *** with your user name).
The "C:\Users\***\AppData\Roaming\Winamp\Plugins\ml" folder is where the media library playlists are stored. Their names begin with "plf" and use the "m3u8" extension.
Playlists you create yourself with the playlist editor can be placed in any folder you want and be named anything you want (within the restrictions for naming files in Win 7). You can use the "m3u", "m3u8", or "pls" extension for these lists.
Win 7 Home Premium 64-bit SP1, WA Pro 188.8.131.5281 beta, cPro MPxi_remix skin, 5.1 speaker system