Hi,
In order to change the database mode from ARCHIVELOG to NONARCHIVELOG mode perform the following steps:

1) Shutdown the database
2)Take a cold database backup of all the
datafiles,controlfiles,logfiles(if possible,recommended)
3) In init.ora parameter file add the following parameters
LOG_ARCHIVE_START=true
LOG_ARCHIVE_DEST=
LOG_ARCHIVE_FORMAT=orcl%s_T%t.arc

%T = Thread Number,left zero padded
%t = Thread number,not padded
%S = Log sequence number,left zero padded
%s = Log sequence number,not padded

4) Start the instance and mount the database(dont open it) and at Server Manager enter the follwowing command
SVRMGR> alter database archivelog;

5)Open the database
SVRMGR>alter database open;

6)Check it whether archivelog is set or not with the follwing 2 queries:
sql>select * from v$database;
svrmgr>archive log list

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

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