I´ve been using Ubuntu 11.10 for a couple of months without any problem. However, i´m now unable to login. It happened after i installed Sophos Antivirus. The system got awfully slow after the installation so i tried to shut my laptop down. The system then refused to shut down (nothing happened) so i performed a hard reset. Then, at the login screen after i enter my password a black screen is showed like half a second and the login screen comes everytime back again. This happens also with the guest account and using both Unity and Gnome desktops.

I already deleted the .Xauthority file in the user home folder and nothing happened. I also renamed the home folder (mv /home Backup) and created a new one with proper owner seetings and that didn´t solve the problem either. I already deinstalled Sophos which didn´t help. Since i have little experience with ubuntu i don´t know what to try next. I´m able to login using ctrl+alt+f1 so my password is working....

Any help would be really appreciated.

I experienced this same type of problem on Monday when I installed Ubuntu on my laptop. I was replacing Mandriva 2010 Powerpack to Ubuntu 11.10. I didn't have DVD-Rs available to backup my home folder, so I just set the installer not to format that partition. I used the same user name and password, but when it went to boot for the first time, it would get the login screen (despite the fact that I set it to log in automatically). I would enter the password, and the screen went black, then flashed back to the login screen.

It wasn't until I set a new patition for the home folder that it worked normally. I set the old home folder as /home/mdk so I can get access to it. It's been working since then. From what it says in your message, you did something similar. So I would suggest if it's possible get a spare hard drive and set a space on it for the /home folder. See if that works.