Maybe this (although I am little doubtful)...
Ctrl+P > General Preferences > uncheck "Disable always on top while fullscreen applications are in focus"
Otherwise, are you using any desktop/shell utilities/enhancer/replacement (eg yzshadow, nView, Litestep, Samurize, StyleXP, Windows Blinds, Perfect Menu, Aston shell, etc.)? Default modern skin or a third party skin? Any applications with their own AOT
Older versions of Winamp are not supported. Try upgrading
(5.05 at the time of this reply).