Hi,
INTERNAL is a proxy user which in turns comes to be SYS.In order to change the password of INTERNAL and SYS user perform the following steps

1.Remove the old password file which resides in
$ORACLE_HOME/database directory.

2.With the orapwd utility create a password file which contains the password for INTERNAL and SYS.

orapwd file=c:\orant\dbs\passwd.ora password= entries=10

To change the password for SYSTEM give the following command

Connect as sys
sql>alter user system identified by ;

In case of any help please be free to ask me at [email protected]

Regards,
ROhit Nirkhe,Oracle DBA,OCP 8i
[email protected]