Ubuntu 12.04 LTS: after reinstalling system unable login as administrator, only allows enter as a guest. Reinstall was done to '/' Ubuntu partition, no formatting, with the same name & pass as in original installation. I used this Guide to reinstall Ubuntu.

You should format all partitions except /home.

But reinstallation solved the issue I had before, now all is worked. I need solve this problem with login.

You only had a 'problem', because you corrupted your filesystem, and didn't let the fsck run until completion. A re-install formatted your drive..it didn't 'solve' the issue, anymore than buying a new car will 'fix' another broken car you have.

Also, you're installing 12.04?? Why? The latest is 16.04..you're FOUR major versions behind on a new installation. The best thing you can do is to boot to single-user mode, make a copy of all your data on /home, and wipe/reload to the LATEST version of things. Installing an old OS is pointless..you're just asking for headaches.

I sticked to 12.04 LTS just because Dell support site specified 12.04 LTS as supported Linux version. No other Linux vers. was specified. On which step the filesystem was corrupted, as you mention? I just followed Ubuntu installation wizard steps. I had issue with missing audio before reinstallation, after reinstall Ubuntu, uadio was restored, and video playback also works, without installing additional drivers. In original installation, video playback by default not worked, so it was necessary to install additional Adobe Flash plugin.

..and that matters how, exactly??? Are you going to call Dell for Linux support? Because if so, you need to then call Dell for an OFFICIAL installation disc, and let THEM do the installation to be under official support. Otherwise, there is NO POINT in loading an old system. Also, you can use pretty much ANY Linux distro on ANY laptop, just as easily.
In your OTHER thread where you tried to run fdisk to fix it (?), then just decided to reload.
All this is immaterial. You're not actually solving your problems by reloading an OS, anymore than you're fixing your leaking roof by buying a new house. If you want to reset your administrator password, did you try putting "forgot root password ubutun 12.04" into Google, and reading the VERY FIRST HIT, which is from the official Ubuntu documentation[/B]? And if you're under 'support' with Dell, have you asked THEM??

This is good advice.

No, this is not the problem I've encountered. I asked because I wanted to know how to check file integrity, just to be on safe side.
But I don't said that I forgot my root password, I don't forgot it, Ubuntu don't allow login as admin, once I login it keeps me back in to login screen.(the screen flashes to console, then I'm thrown back to the login splash). If even I mistype pass, error is always visible.
Surely I expected that most likely I will the range of issues with Ubuntu that come preinstalled with laptop, but not so many, and not from the outset. And it will take ages to find the faulty configuration file.

This: http://www.linuxquestions.org/questi...ty-4175585031/

...is that thread, where you force-stopped the upgrade. In that very thread you say that you used recovery to factory-state, right??? What do you think that does? You corrupted your system by doing something bad, had to reset to 'factory' (i.e. initial state). Fsck will report any bad spots on the screen...there is no saved report. Read the man page. But from THAT thread (your PREVIOUS thread), we're here to your NEW thread, which is titled

Re-installation...but you didn't 'reload'??? You do realize they mean the same things, right???
First, DO NOT LOG IN AS ROOT/ADMINISTRATOR, PERIOD. That's been covered on here as a VERY bad thing thousands of times, and there isn't ANY site that will tell you this is good. Log in as your regular user, and assume root ONLY when needed to. Why do you think that administrator/root login is disabled BY DEFAULT???

Secondly, again...did you try just putting "enable root login ubuntu 12.04" into Google? Read the "Question Guidelines" link in my posting signature. The FIRST TWO HITS are:
https://help.ubuntu.com/community/RootSudo
http://askubuntu.com/questions/44418...ble-root-login

...which are LOADED with warnings about NOT doing this, for very good reasons.
You're having issues, because you are doing things badly. This isn't because of Linux...it's you. You're manually stopping things that SHOULD NOT be stopped, running as root, etc. Also, you've now been using Linux for two years...why are these things 'new' to you?

AGAIN...if you have a Dell laptop that's under support with Dell, using Dell supplied OS, WHY are you not CONTACTING DELL FOR HELP??? If you're NOT under support, stop trying to force things to work in ways they're not supposed to, stop where you are, and load the latest versions of things, and move forward. You're having problems because you CAUSED them with the botched upgrade. If you HAD admin/root login enabled before, and you reinstalled, why are you amazed that it's not there now? You reset the system, which obviously puts that back, right????

I have not been using Linux for a very long time now, so I forgot many details, include important things about root account. Can I upgrade to Ubuntu 14.04.4 from guest account? If so, should this solve all login problems?

I can also probably install another Linux distrbutive, something stable, for example last Debian pack, but I would preffer distributive with preinstalled graphical desktop and minimal set of handy applications like Ubuntu have (LibreOffice, etc)

I'm not sure how should I format disk or set new partitions to install new pack, or I can install new distributive to /sda3 in place of Ubuntu, without touching other partitions?

Since you cancelled an earlier upgrade that did exactly that, is it not OBVIOUS THAT YOU CAN??? And AGAIN, since you keep mentioning 'support' from Dell, have you CALLED THEM???
AGAIN, you can load ANY version of Linux. For you, I think Mint would be best. ANY of them, have graphical desktops, and would probably install in about 30 minutes.
AGAIN you should back up your data on /home (or wherever you keep your personal files), and FORMAT EVERYTHING. When you go to install a new distro, let it partition your disk for you. That's it.

Not sure how many times we have to say the same things.....

I can login as Administrator use TTY4, but not from GUI. I upgraded to Ubuntu 14.04 from TTY console. But I'm still can no login from GUI as admin. I have not set root priveleges, that configuration was set by default, from Dell. Also, when I reinstalled 12.04 I only repeated same steps that was required during very first configuration. Dell phone support is not available in my area, btw.
How to reset access to Administrator account via GUI?
If there is some file corrupted, the only way to solve the issue is completely reinstall system?
Windows have many system diagnostic and repair utilites that can solve various issues, but Linux lack this functionality.

Linux has many system diagnostic and repair utilites too, but they require relevant knowledge. Unlike Windows, Linux is not designed to be idiot-proof.

What's the output from this command?

(It doesn't make any changes, it just informs you about potential issues with packages.)

In terms of usability, for most of general users, Windows is still far ahead of Linux, unfortunately.

sudo apt-get -f install

if run from Guest account, it returns

from tty4:

In that case, maybe you should use Windows instead of Ubuntu.