Rename isc4.gdb to something else, and install fresh isc4.gdb
(e.g. from installation). Then log to renamed database and change
the password to standard 'masterkey':

UPDATE USERS
SET PASSWD='n2zpJI3qBh2'
WHERE USER_NAME='SYSDBA'

Then shuffle the files back (remove isc4.gdb and rename
renamed-and-sysdba-password-updated-isc4 to isc4.gdb).

Ivan
http://www.volny.cz/iprenosil/interbase