I had installed ubuntu 12.10 with security key for encrypting system hard disk which requires you to enter the passphrase before the login screen every time you boot your system... i wanted to get rid of that security key so i used a command in terminal while my system was working fine but somehow the command disabled the logon to my system (even after using the correct PW same is not opening ... i wanted ti reinstalll ubuntu through bootable USB but i think the security key for encrypting the hard disk is preventing the same ... i am just left with a useless laptop with me where i cant even install new OS .... what to do please help....how to disable the security key encryption passphrase or atleast how can i access or logon to my system Please help ...
As long as your hard drive doesn't support encryption itself and you used it there is no way an encrypted partition can prevent a simple reinstall.
So you should have no problems booting from USB and installing ubuntu again. Maybe go to the manual partitioning and remove all partitions from disk during setup, but this shouldn't be necessary at all if you choose guided using the whole disk.
You should be able to boot from usb or cd even if disk is encrypted(as the encryption passphrase is asked for after the HD boots).
Make sure to go to your bios settings and double check that they have priority over your HD. Likewise you could try using Dariks boot and nuke (http://www.dban.org/) to wipe your system and then run your Ubuntu installer.