From last week all the window on Mint 14 Xfce has no title bar. There is no background image. No icons on task bar. Some shortcuts are invalid, including Alt+Drag(move window), Alt+F9(minimize), Alt+F4(close window), etc.
Solution:
In terminal:
xfdesktop &
taking back the background image;
xfwm4 &
taking back window border.
bingo.