I did following
Step 1 First update your repository by running
sudo apt-get update
Step 2 There is some bug in old version of lightdm, so we need to upgrade the same. To do so run,
sudo apt-get install lightdm
Step 3 Now we have to modify grub config.
Step 3a Open /etc/default/grub with your faviourite editor and change
Step 3b Also comment GRUB_HIDDEN_TIMEOUT=0 This line is for unhiding the GRUB menu
Step 4 Now we will upgrade GRUB configuration
Step 5 Ubuntu 11.10 Desktop edition use lightdm for GUI. We need to disable the same
sudo update-rc.d -f lightdm remove
Step 6 Now restart your machine.
Found at Disable GUI Boot in Ubuntu 11.10