ORA-01031: insufficient privileges error

>sqlplus / as sysdba
SQL*Plus: Release 10.1.0.2.0 - Production on Thu Sep 20 10:51:57 2012
Copyright (c) 1982, 2004, Oracle. All rights reserved.
ERROR:
ORA-01031: insufficient privileges


ORA-01031: insufficient privileges error occurs when you attempt to execute a
program or function for which you have not been granted the appropriate privileges.

Solution1 :
Make sure that oracle user is a member of the ORA_DBA group on window and
dba group (e.g. /etc/group) on linux.

 Solution2 : 

If you have already added the oracle user in above group then check have you installed
any other oracle component like owb, in different home then you must have to set the proper environment variable.

verify to connect using complete Path if it connected

>E:\oracle\product\10.2.0\db_1\bin\sqlplus / as sysdba
SQL*Plus: Release 10.2.0.1.0 - Production on Thu Sep 20 10:53:48 2012
Copyright (c) 1982, 2005, Oracle. All rights reserved.
Connected to:

If you are able to connect then set ORACLE_HOME properly