Not exactly a solution but try this:
Log into your desktop and wait until the hard drive stop reading. The way you describe it seems like unity is not loading properly.
After this, switch and log into a tty. You can do this pressing Ctrl+Alt+F6 and log into the terminal.
Change the variable DISPLAY so the next commands will affect the graphic environment:
compiz --replace &
This should make compiz try to restart. You should return to the GUI (Ctrl+Alt+F7 or Ctrl+Alt+right arrow) and see if it loads the desktop. Any error message would show up in the tty6 you executed the command.
If this doesn't work, try to reset unity to defaults, in the same tty6:
And repeat the process, try to reload compiz/unity again and see if it works. As a last advice if this is not working you could install an alternative desktop manager like gnome-shell or xfce:
sudo apt-get install gnome-shell
sudo apt-get install xubuntu-desktop
That way you'll have at least an usable desktop to surf the web and search the answer. It may be a problem with your graphic drivers and after installing them it may work. What graphic card are you using? I remember having a lot of problems with ATI cards...
Wish it helps, good luck!