Thanks! I think I have everything mentioned in this KB note(see my original post), but it still doesn't work. Actually, you don't have to create ORA_DBA group manually (at least to me). It got created by installer when you install Oracle.

Originally posted by gnagesh
Try this may help you:

From Metalink Note:50507.1

OSDBA & OSOPER Groups on NT
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The 'OSDBA' and 'OSOPER' groups on NT are simply groups with the name "ORA_DBA", "ORA_OPER", "ORA_sid_DBA" or "ORA_sid_OPER", where 'sid' is the instance name.
Eg: To make a user an administrative user simply:
a) Ensure there is a line in the SQLNET.ORA file which reads: SQLNET.AUTHENTICATION_SERVICES = (NTS)
b) Create a LOCAL user
c) Create a local NT group ORA_DBA or ORA_sid_DBA where 'sid' is in upper case
d) Add the user to the ORA_DBA or ORA_sid_DBA group
e) That user should now be able to "connect / as sysdba"

Hope this helps.