You haven't mentioned your ubuntu version and your window manager (unity 3d/compiz, unity 2d, metacity/gnome classic etc.)

Steps

1. Backup your important data.




2. Please create a new user, then grant admin privileges and check if it works there. If it works, then the problem is somewhere in your user's home directory. Some program settings must have been altered.




3. (a) If you also have problems with the new user, try purging packages:

   
   
   

   sudo apt-get update
   
   sudo dpkg -P --force-all ubuntu-desktop ubuntu-standard ubuntu-minimal
   
   sudo apt-get install ubuntu-desktop ubuntu-standard ubuntu-minimal
   
   sudo apt-get -f install
   
   sudo apt-get dist-upgrade
   
   

   
   
   

   This is mostly harmless and will install any important packages you may have removed.
   Restart your pc and see if it's fixed.

   
   
   

   (b) If you do not have any problems with the new user, then some settings in your home configuration are causing trouble. Close any windows that you don't need and do not open anything.

   
   
   

   The following commands will reset your gconf and dconf settings. This includes window positions, default theme, desktop wallpaper/backgrounds, only very basic preferences in programs (e.g. nautilus will be in default view, not list view). but NOT your data nor any folder-specific settings.

   
   
   

   There is a small chance that something will go wrong, so please backup (#1) and have a live cd/usb somewhere in case you need to reinstall. Or use your new administrator account to fix the issues.

   
   
   

   The commands are:

   
   
   

   gconftool --shutdown
   
   killall -r -I -9 dconf
   
   killall -r -I -9 gconf
   
   mv .gconf gconf-backup
   
   mv .config/dconf config-dconf
   
   mv .cache/dconf cache-dconf
   
   

   
   
   

   Then reboot (do not do anything else, do not open/close any windows):

   
   
   

   sudo reboot
   
   

Recovery

To recover your old settings:

gconftool --shutdown

killall -r -I -9 dconf

killall -r -I -9 gconf

mv gconf-backup .gconf

mv config-dconf .config/dconf

And reboot (do not do anything else, do not open/close any windows):

sudo reboot

If it didn't work

• Did you by any chance install packages from any PPA or third-party repositories?

  
  
  

  These packages are not all checked, nor are all safe. Some exist for testing purposes only. I suggest you reinstall your system without any PPA until you become acquainted with the linux system (and how to recover).