Sorry for the confusion as it was end of the day i was little hurry in working and pasting the things.Here are the latest results
1.On a server where only one instance is there.Remember even i didn't set the SID.

Microsoft Windows 2000 [Version 5.00.2195]
(C) Copyright 1985-2000 Microsoft Corp.

C:\Documents and Settings\akhadar>cd\

C:\>sqlplus /nolog

SQL*Plus: Release 8.1.7.0.0 - Production on Wed Apr 16 07:27:46 2003

(c) Copyright 2000 Oracle Corporation. All rights reserved.

SQL> conn / as sysdba
Connected.
SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------

RICT.CRB.SE.COM.SA

SQL> exit
Disconnected from Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production
With the Partitioning option
JServer Release 8.1.7.0.0 - Production

C:\>

2.On the server where there are 2 instances

Microsoft Windows 2000 [Version 5.00.2195]
(C) Copyright 1985-2000 Microsoft Corp.

C:\Documents and Settings\akhadar>cd\

C:\>set oracle_sid=ricd

C:\>echo %oracle_sid%
ricd

C:\>sqlplus /nolog

SQL*Plus: Release 8.1.7.0.0 - Production on Wed Apr 16 07:30:40 2003

(c) Copyright 2000 Oracle Corporation. All rights reserved.

SQL> conn / as sysdba
ERROR:
ORA-12560: TNSrotocol adapter error


SQL> conn /@ricd as sysdba
Connected.
SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------

RICD.CRB.SE.COM.SA

SQL> disc
Disconnected from Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production
With the Partitioning option
JServer Release 8.1.7.0.0 - Production
SQL> exit

C:\>set oracle_sid=ricd1

C:\>echo %oracle_sid%
ricd1

C:\>sqlplus /nolog

SQL*Plus: Release 8.1.7.0.0 - Production on Wed Apr 16 07:31:59 2003

(c) Copyright 2000 Oracle Corporation. All rights reserved.

SQL> conn / as sysdba
ERROR:
ORA-12560: TNSrotocol adapter error


SQL> conn /@ricd1 as sysdba
Connected.
SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------

RICD1.WORLD

SQL> disc
Disconnected from Oracle8i Enterprise Edition Release 8.1.7.0.0 - Production
With the Partitioning option
JServer Release 8.1.7.0.0 - Production
SQL> exit

C:\>