How to reinstall ubuntu after the first installation fails
 What to do if an installation fails in Ubuntu
If you stop an installation half-way through, a lock file is created that prevents you from installing anything else. Before you can try again you need to manually unlock the download cache.
1. Open a root terminal and type nautilus /var/cache/apt/archives
2. Right click on the file called lock and delete it
That should sort the problem out.