I have removed the mentioned libraries. Then my system disconnected from NIS service and many packages are being removed. The problem is I am not even able to login in my system. When i restarted it is showing "low graphics mode error". I do not know what to do Please help me. I am a beginner to Ubuntu.
Just this one made your system unusable:
Package: libc6 (2.15-0ubuntu10.5) [security]
Embedded GNU C Library: Shared libraries
Contains the standard libraries that are used by nearly all programs on the system. This package includes shared versions of the standard C library and the standard math library, as well as many others.
I would suggest to re-install without(!) formatting discs. It is the easiest and quickest method to get your system back up. In case you have files on that system you need to save: use the live dvd to mount your user partition and burn them to a dvd. Just as a precaution.