Then, restart your computer, enter the "recovery mode" in your start-up, and when you are in there, select the option where you can use the command line as "root". From there (note this down), you just type:
``nano /etc/hostname``
then, when you have, enter a pc name / hostname like "computer", and exit with ctrl+x or what it says, to save it. reboot, and try the things again :P