ora-28000 the account is locked

C:\oraclexe\app\oracle\product\10.2.0\server\BIN>sqlplus.exe / as sysdba

SQL*Plus: Release 10.2.0.1.0 - Production on Mon Jan 19 17:01:35 2009

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Connected to:
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Production

SQL> alter user system identified by password;

User altered.

SQL>