ORA-12557: TNS:protocol adapter not loadable

Problem Description
In my machine I had oracle 10g home , using sqlplus of 10g I could connect to an Oracle database 10g. Now I have installed a new oracle 11g home, but using sqlplus of 11g I could not connect to Oracle database 10g. Below is an example,

With 10.2g sqlplus I can connect to 10g database.
C:\>e:\oracle\product\10.2.0\db_1\bin\BIN\sqlplus.exe maximsg/a@192.168.100.160/tiger

SQL*Plus: Release 10.2.0.1.0 - Production on Mon Jan 26 01:54:10 2009

Copyright (c) 1982, 2005, Oracle. All rights reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

With 11g sqlplus I could not connect to oracle database 10g. It returns error message, ORA-12557: TNS:protocol adapter not loadable.
C:\>d:\app\oracle\BIN\sqlplus.exe maximsg/a@192.168.100.160/tiger

SQL*Plus: Release 11.1.0.6.0 - Production on Mon Jan 26 01:55:00 2009

Copyright (c) 1982, 2007, Oracle. All rights reserved.

ERROR:
ORA-12557: TNS:protocol adapter not loadable

Cause of the Problem
The problem happens because of two ORACLE_HOME are installed on your system. As after 10g you hav e installed 11g so whenever you write sqlplus by default new 11g binaries are selected and raises ORA-12557. But working with old home 10g works fine.

Simply sqlplus does not work but 10g home location sqlplus (e:\oracle\product\10.2.0\db_1\bin\BIN\sqlplus.exe ) works.

C:\>sqlplus.exe maximsg/a@192.168.100.160/tiger

SQL*Plus: Release 11.1.0.6.0 - Production on Mon Jan 26 22:47:08 2009

Copyright (c) 1982, 2007, Oracle. All rights reserved.

ERROR:
ORA-12557: TNS:protocol adapter not loadable

Connecting to old oracle database using new binaries are not supported in oracle and error will return.

Solution of the Problem
Only setting ORACLE_HOME is not sufficient on windows environment. Because the location is taken from windows registry. So either uninstall newer oracle home or explicitly pointing to old oracle binaries will solve the problem.

Here using pointing to old home,
C:\>e:\oracle\product\10.2.0\db_1\bin\BIN\sqlplus.exe maximsg/a@192.168.100.160/tiger

SQL*Plus: Release 10.2.0.1.0 - Production on Mon Jan 26 01:54:10 2009

Copyright (c) 1982, 2005, Oracle. All rights reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

Alternatively you can follow below.

Step 01: Remove registry entries of new ORACLE_HOME.
To do this,
i)Type regedit on Run.
ii)Press enter and expand HKEY_LOCAL_MACHINE.
iii)Then expand SOFTWARE and then expand ORACLE tab. There you will see two oracle home. Right click on the one that you want to delete and then select delete. If prompting click yes.

Step 02: Remove any environmental variable.
i)Right click on My computer icon. Then select properties.
ii)System properties window will appear. Click on Advanced tab.
iii)Select environmental variables.
iv)Find the variable/system variable path and ORACLE_HOME. Edit or modify them so that it point to you desired sql*plus.
Usually in the PATH system variable you will get both ORACLE_HOME path. Just remove one path. Of course if you have ORACLE_HOME variable settings first delete the key.