Hi friends...
When i try to enter into root user it shows the following error msg.. "The System administrator is not allowed to login from this screen " so what shall i do...?

More information please.
Which tool or application are you using to try to login as root?
If you are using command-line, what command do you give which results in the error message?

The majority of distros do not allow root logins at the login screen by default. There is a way around that, but it is usually not necessary nor recommended on an installed system. Some live CDs allow this because you will not destroy the CD, and many CD distros are used for system repair, partitioning, and forensics on the installed system.

If you were the first user assigned to the system, then you were assigned as the administrator, and have SU (root or 'Super User') permissions when performing a task as SU and entering the root password either in a terminal or a pop-up window. If you are a subsequent user, then the administer must add you to the 'sudoer' list granting the same priviledges.

So were you the first user assigned to the system? And are you referring above as attempting to login at the opening 'Login Screen' or trying to open an application as the root user?