SYS is a very special user account. It has been associated with the highest privilege call SYSDBA. Normally, you should not connect to the server with SYS. But if you want to use it, you need to use a special connect command:

>cd (OracleXE home directory)

>.insqlplus /nolog

SQL> connect SYS/globalguideline AS SYSDBA

Connected.

SQL> quit

Note that the "/nolog" option is used to start SQL*Plus without login immediately. A special form of the "connect" command is used to include the user name, password, and the privilege in the same line.

You can not log in with SYS without SYSDBA privilege.