I was using Ubuntu 11.10 and installed the KDE Desktop and I was happy with that. But I wanted to get my old login screen back so I removed the package kdm. When I boot my PC now, it keeps booting and nothing happens.

How can I add the kdm package again? I got into recovery mode, terminal and tried to execute sudo apt-get instal kdm but I got an error message.

W: Not using locking for read only lock file /var/lib/dpkg/lock

E: Unable to write to /var/cache/apt/

E: The package lists or status file could not be parsed or opened

I'm pretty desperate now, could somebody please help me?