How to convert NOARCHIVELOG to ARCHIVELOG

C:\Documents and Settings\Administrator>sqlplus / as sysdba

SQL*Plus: Release 10.2.0.3.0 - Production on Tue Jan 18 13:36:02 2011

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Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> SELECT log_mode FROM v$database;

LOG_MODE
------------
NOARCHIVELOG

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area  289406976 bytes
Fixed Size                  1290208 bytes
Variable Size             201326624 bytes
Database Buffers           83886080 bytes
Redo Buffers                2904064 bytes
Database mounted.
SQL> alter database archivelog;

Database altered.

SQL> alter database open;

Database altered.

SQL> SELECT log_mode FROM v$database;

LOG_MODE
------------
ARCHIVELOG

SQL>