cannot open "applications'" in the menu bar

I am using ubuntu 64 bit on a celeron 3.06 pc. i cannot open "Applications" on the menu bar, though i can open "Places" an d"System". The problem continues if i add the menu bar to panel at the base. In order to trouble shoot i typed in alacarte in the terminal and received the following error message:

/usr/lib/python2.5/site-packages/apt/progress.py: inconsistent use of tabs and spaces in indentation
Traceback (most recent call last):
  File "/usr/bin/alacarte", line 36, in <module>
    main()
  File "/usr/bin/alacarte", line 32, in main
    app = MainWindow(datadir, version, sys.argv)
  File "/usr/lib/python2.5/site-packages/Alacarte/MainWindow.py", line 49, in __init__
    self.editor = MenuEditor()
  File "/usr/lib/python2.5/site-packages/Alacarte/MenuEditor.py", line 36, in __init__
    self.__loadMenus()
  File "/usr/lib/python2.5/site-packages/Alacarte/MenuEditor.py", line 46, in __loadMenus
    self.applications.dom = xml.dom.minidom.parse(self.applications.path)
  File "/usr/lib/python2.5/site-packages/_xmlplus/dom/minidom.py", line 1915, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python2.5/site-packages/_xmlplus/dom/expatbuilder.py", line 926, in parse
    result = builder.parseFile(fp)
  File "/usr/lib/python2.5/site-packages/_xmlplus/dom/expatbuilder.py", line 211, in parseFile
    parser.Parse("", True)
xml.parsers.expat.ExpatError: no element found: line 1, column 0

I cant understand the meaning of this error message.
Any help would be appreciated.

Jatan