Try these steps, from the command line:
Fix broken dependencies:
$ sudo apt-get install -f
If it still doesn't work, try this to fix/reconfigure the existing, unpacked-but-not-yet-configured packages:
$ sudo dpkg --configure -a
Then try the upgrade again.
If it fails again, you may want to delete all the already- downloaded packages (they should be stored in
$ sudo apt-get clean
That will clean up the packages but if you're still stuck in some weird state, I'm not sure what else to suggest.
Of course, before you do anything you should back up any important contents of your
/home/dave directory (or whatever your $HOME dir is). Hopefully you won't have to do a complete re-installation but at least you'd have your data.