Major Error of installing updates

Please try the following from a terminal:

gksudo synaptic

this should start it with administrator privileges and ask you for password. Let me know whether it works.

No sorry. It doesn't work. When I tried it before, it asked for my password as it should, but when I ran Update Manager, it still gave error messages.

I have also tried sudo synaptic.

Just now I tried Alt +F2 and ran gksudo synaptic, the place to type my password comes up, but afterwards I get this message:
"Failed to run synaptic as user root.

The underlying authorisation mechanism (sudo) does not allow you to run this program. Contact the system administrator."

Can You try booting in the recovery mode and see whether it works?

Synaptic isn't in my Menu anymore as well as loads of administrative programs. I can't use any programs which require a administrator's password.
I'll try Recovery Mode.

Nope it didn't work.

Didn't work in recovery mode. Don't know what to do.

If I'm reading this correctly, you can't do anything as an administrator, that is to say it's not limited to package management.

Can you tell me any output from the following:
1. In a terminal window, type
       sudo -i
    This should ask for your password, and if it works correctly, you should end with a [root@yourhost:~]# <-- note the # on the end, not the normal $. If it did work, ignore #2 and #3 below, and post back here that it worked.

2. If that fails, can you tell me the output of the command:
     groups yourusername

3. Finally, do the following command to grab the dates
     ls -l /etc/pam.d/

Ok. The Output of that command was : amarpreet adm dialout fax cdrom floppy tape audio dip video plugdev scanner netdev lpadmin powerdev fuse

Number 1 didn't work - it said: "Sorry, user amarpreet may not run sudo on amarpreet-desktop". Would creating a new user work? If it would, how can I create a new user without administrative privileges?

You will need t get yourself into the admin group.

When you turn the computer on, in the grub screen (where you choose recovery mode or not), do the following:
1. Press Esc to stop the countdown to regular boot
2. Move down to one of the recovery items
3. press e
4. move down the the line that looks like 'kernel /vmlinuz...', and press e again
5. you should see at the end of the line something like 'ro single'. After that, type init=/bin/bash
6. press enter, which should return you to the list with 'root', 'kernel', 'initrd'. From here, press b to boot
7. This is now a read-only recovery console, which is very limited. The first step will be to make it writable. type mount -o remount,rw /

The above instructions will put you to a root command prompt without a password (this is useful if you've forgotten your password)

Now, to add you to the admin group:
using your favourite text editor (pico?), we will edit /etc/group.
First, take a backup
cp /etc/group /etc/group.bak
Next, edit it
pico /etc/group

find the line that starts with admin (mine is)
admin:x:110:myname,anothername,anothername

add your name to the end of that line. Save and quit and you should be good to go.

Just for good measure, while you are in the recovery console you should check the sudoers file, and ensure 'admin' is still the group that does have the right to do sudo, and set the root password so this is is easier if it happens again!

Check the sudoers file:
** Be very careful on any change to this - breaking this file can render your system unusable **
use your favourite editor to edit the file /etc/sudoers
you should find a line (most likely the last line) that reads exactly like this:
%admin ALL=(ALL) ALL

%admin means the admin group, so you are good to go.

Set the root password:
type passwd. It will ask you for the password twice. In the future, if you've forgotten your own password, or somehow you are not in the admin group anymore, you can use the root account to fix these things.

Rebooting:
type reboot or press ctrl+alt+del