Login into the machine as oracle login user id( where oracle is installed)..
Add ORACLE_HOME = <Oracle installation Directory>
in Environment variable
Open a command prompt
Change the directory to %ORACLE_HOME%\bin
type the command sqlplus /nolog
SQL> connect /as sysdba
SQL> alter user SYS identified by "newpassword";
One more check, while oracle installation and database confiuration assistant setup, if you configure any database then you might have given password and checked the same password for all other accounts.. If so, then you try with the password which you have given in your database configuration assistant setup.
Hope this will work for you..