ORA-01031: insufficient privileges with sqlplus /AS SYSDBA

C:\>sqlplus "/AS SYSDBA"
SQL*Plus: Release 10.2.0.3.0 - Production on Tue May 6 04:42:32 2008
Copyright (c) 1982, 2006, Oracle. All Rights Reserved.
ERROR:
ORA-01031: insufficient privileges
Enter user-name:

If you are getting the above error when you try to login with "sqlplus /AS SYSDBA", please check if the following are properly set.
1] The Windows user ID with which you signed in is part of the ORA_DBA group.
2] SQLNET.AUTHENTICATION_SERVICES=(NTS) in sqlnet.ora file.

Now try again.....

C:\>sqlplus "/AS SYSDBA"
SQL*Plus: Release 10.2.0.3.0 - Production on Tue May 6 04:49:13 2008
Copyright (c) 1982, 2006, Oracle. All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Production
With the OLAP and Data Mining options
SQL> show user
USER is "SYS"