freeing space on computer

For your information:

On Ubuntu normal users are granted basic privileges only, without the authority to do system-wide changes.
For system-wide changes you have to use administrative authorizations.

If your user is member of the admin group, you can temporarily enable administrative rights by putting sudo (for terminal commands) or gksudo (for graphical programs) in front of the command.
You have to be aware, that you will be asked for your password as confirmation, and for sudo you will not see anything on screen when you type the password, no echo, not even '*' placeholders.

So I guess the "sudo apt-get clean" command did in fact work, but it seem that in your case it did not help much.

To get some more information about your system please do the following:
open a terminal (e.g. by simultaneously pressing ctrl-alt-t) and issue the following commands

uname -a
lsb_release -a
sudo apt-get autoremove
df -h
df -I
dpkg -l | grep linux-head
dpkg -l | grep linux-kern

then select all output (by using the terminal window's menu entry "edit - select all"), copy it ("edit - copy") and paste into this question document for us to see.

Also install and run bleachbit as both root and your user. Be sure to use carefully and avoid options which say they will take a long time.

I am the main and only user- it is a personal computer. So should the commands be different to do a 'get clean'?

Your user is a user and nothing more. You are in a special group which gives you the ability to run sudo and gksudo which then gives you the ability to run commands as root.

Can you please run the commands Manfred gave and provide the output.

Thanks

~$ uname -a
Linux Multimedia1 3.2.0-51-generic #77-Ubuntu SMP Wed Jul 24 20:21:10 UTC 2013 i686 i686 i386 GNU/Linux
goodwill@Multimedia1:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 12.04.2 LTS
Release: 12.04
Codename: precise
goodwill@Multimedia1:~$ sudo apt-get autoremove

I hope no one will hack into my computer now- is there any sensitive info here?

Ok and what is the output of :

dpkg -l | grep linux-image

Thanks

rc linux-image-2.6.32-40-generic 2.6.32-40.87 Linux kernel image for version 2.6.32 on x86/x86_64
ii linux-image-2.6.32-41-generic 2.6.32-41.90 Linux kernel image for version 2.6.32 on x86/x86_64
ii linux-image-3.2.0-31-generic 3.2.0-31.50 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-3.2.0-32-generic 3.2.0-32.51 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-3.2.0-33-generic 3.2.0-33.52 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-3.2.0-34-generic 3.2.0-34.53 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-3.2.0-35-generic 3.2.0-35.55 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-3.2.0-37-generic 3.2.0-37.58 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-3.2.0-51-generic 3.2.0-51.77 Linux kernel image for version 3.2.0 on 32 bit x86 SMP
ii linux-image-generic 3.2.0.51.61 Generic Linux kernel image
goodwill@Multimedia1:~$

sudo dpkg -P linux-image-2.6.32-40-generic
sudo apt-get --purge remove linux-image-2.6.32-41-generic
sudo apt-get --purge remove linux-image-3.2.0-3*
sudo apt-get --purge autoremove

Will leave you with one kernel.