something weired about my Windows manager

[loody]

Dear all:
I use xubuntu and the windows manager seems get wrong setting.
I list my problems below:
1. usually when I click the right-up corner button of xubuntu.
    It will pop out a dialog and ask me whether I need to turn off,
Hibernate, reboot, etc. the computer.
    But it only shows:
    Exit Xfce panel?

2. usually when I click the right button of the mouse on the desktop,
it will pop out a list and show all the programs I can run.
But it disappears right now.

3. it will have firefox button at the top of screen, but it disappear right now.

Is there any fast way to reset my destop setting, except re-install xubuntu?
appreciate your help,
miloody