Admin password authenticates to log in, but not to install apps or change user accounts

I just installed Ubuntu 12.04. Then I changed my own user profile to not require a password and log me in automatically. What happened is, I'm still asked for a password once the desktop diplays. Once I'm in, I find my password is not accepted when I'm trying to install an application or make changes to user accounts.

I'm still listed as an administrator, so I figure something must have happened to my password. My question is, why is my password accepted for logging in as a user, but not to do administrator stuff? It's the same password, isn't it? The one I set when I installed Ubuntu.

I tried resetting the password at the root shell prompt (accessed via recovery mode), but I only get an error, with the result "Password unchanged".

Help! Is there something else I can do to avoid completely reinstalling Ubuntu?

I had the same issue. Changing, just a bit, Robin's answer solved the problem...

I've changed a little the steps suggested on: http://www.psychocats.net/ubuntu/fixsudo

1. Boot in recovery mode as root
2. Enter the command: mount -o rw,remount /
3. Enter the command: groupadd admin
4. Enter the command: adduser username admin
5. Enter the command: sudo nano /etc/sudoers
6. Right below root ALL=(ALL:ALL) ALL add this line username ALL=(ALL:ALL) ALL
7. Press Ctrl+X and answer 'y' to save the changes
8. Enter the command: passwd username
9. Enter your user's password
10. Enter the command: exit
11. Resume normal boot
12. You 've made it!!!!

*where username is your actual username.