run comuter-janitor in terminal

see the man pages

man computer-janitor

or visit http://manpages.ubuntu.com/manpages/precise/en/man8/computer-janitor.8.html

Maybe you have to use sudo to obtain the required administrative authorizations.

sudo computer-janitor find

etc.

Could also try bleachbit. Be VERY careful with both tools

tried "sudo computer-janitor find"

Got error
"ERROR:dbus.proxies:Introspect error on :1.65:/: dbus.exceptions.DBusException: org.freedesktop.DBus.Error.AccessDenied: Rejected send message, 1 matched rules; type="method_call", sender=":1.182" (uid=0 pid=9068 comm="/usr/bin/python /usr/sbin/computer-janitor find ") interface="org.freedesktop.DBus.Introspectable" member="Introspect" error name="(unset)" requested_reply="0" destination=":1.65" (uid=0 pid=2527 comm="/usr/bin/python /usr/share/computerjanitor/janitor")
 I'm lost!

Please provide some more Information about your system, and what you want to achieve with Ubuntu-janitor.

What is the output of the terminal commands

uname -a
lsb_relase -crid
computer-janitor --version
python --version
dpkg -l | grep computer-janitor

Simply want to clean up all the dead files and bits.

ross@ross-netbook:~$ uname -a
Linux ross-netbook 3.2.0-76-generic #111-Ubuntu SMP Tue Jan 13 22:16:36 UTC 2015 i686 i686 i386 GNU/Linux

ross@ross-netbook:~$ lsb_relase -crid
No command 'lsb_relase' found, did you mean:
 Command 'lsb_release' from package 'lsb-release' (main)
lsb_relase: command not found
ross@ross-netbook:~$ lsb_release -crid
Distributor ID: Ubuntu
Description: Ubuntu 12.04.5 LTS
Release: 12.04
Codename: precise

ross@ross-netbook:~$ computer-janitor --version
Computer Janitor 2.1.0
ross@ross-netbook:~$ python --version
Python 2.7.3

ross@ross-netbook:~$ dpkg -l | grep computer-janitor
ii computer-janitor 2.1.0-0ubuntu8 Clean up a system so it's more like a freshly installed one
ross@ross-netbook:~$

Thanks for your help, I'm not good with xubuntu

Ross.!

Try bleachbit, it can help clear space. Have as many applications closed as possible.

Avoid options that say they will take a long time and be careful of the options you choose as it may remove settings you want to keep (like web browser customisations).

Run it as your user, then as root for a really deep clean, the application is launched from the usual menu (You will need to enter your password in order to run it "as root" as it uses gksudo).