Can't log in as admin

Hello! I have a Macbook Pro Early 2009, Mac OS X Mountain Lion and I am facing the following problem:
After downloading (from the official website, not from App Store) and while installing ClamXav, I tried opening Safari, but it said that some necessary plugin was missing and that I had to re-install Safari, even though 1 minute before that it was functioning normally. The menu bar became unresponsive and I couldn't even force quit the ClamXav installation. I pressed the shut-down button and when I turned my MBP on again, it wouldn't log in as an administrator (note: I installed ClamXav as an admin), but it would look as if it were loading and then freeze, so I had to hold down the shut-down button again. However, I could log in as a Guest User and that's how I uninstalled ClamXav using AppCleaner (though I think it only deleted the app from the guest user account) and later ran Disk Utility to Repair Permissions. No luck. I tried logging in in safe mode, but I still wasn't able to log into my account. I left it loading for about 40 minutes and it never logged in.

Any suggestions? Thanks in advance!

You mean that whenever I can't get it to shut down from the Menu Bar, I should press Cmd-Ctrl-Power Button to turn it off safely? Thanks for the advice!

I have tried the disk repair (didn't make any difference) and hardware test (everything appeared to be OK).

Won't reinstalling the OS delete any files? And besides, my MBP came with a different OS than I'm currently using, as it has been updated.

Time Machine, yep.

Launching Activity monitor and after switching to "All Processes", I noticed that there's a "kernel_task" running through the root user.

PID: 0
Process Name: kernel_task
User: root
%CPU: 0.9-2.5 (flunctuating)
Threads:77
Real Mem: 415.0 MB
Kind: Intel (64-bit)

Isn't root my administrator account? Since I cannot log in to that, how come this kernel task is running? There's more processes running at the root user as I can see in the activity monitor, but this is too big considering that I haven't even logged in there. Could this kernel task be the problem? I suppose not. And why can't I quit it?

And anyway, is it possible that one of the tasks shown in Activity Monitor may be what's preventing me from logging in as admin?

I figured that, yes. Maybe the only solution is to backup the disk and then reinstall OS... I can't find any other way.