Oracle database 11g R2 client sqlplus cannot connect to the database

Problem Environment:
1. Database version: Oracle Database 11g Enterprise Edition Release 11.2.0.1.0-64bit Production
2. Operating System Environment: Windows 7 flagship edition 64-bit
3. Four Oracle database software (four database software, not four databases) are installed locally as follows:
Oracle 10.2 server software: e: \ oracle \ product \ 10.2.0 \ db_1 \ BIN \ sqlplus.exe
Oracle 11.2 server software: e: \ oracle \ product \ 11.2.0 \ db_1 \ BIN \ sqlplus.exe
Oracle 10.2 client software: e: \ oracle \ product \ 11.2.0 \ client_1 \ BIN \ sqlplus.exe
Oracle 11.2 client software: e: \ oracle \ product \ 10.2.0 \ client_1 \ BIN \ sqlplus.exe

Problem description:

1. Start Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 and the service name is sun. The service status is as follows:
C: \ Windows \ system32> SC query oracleservicesun
SERVICE_NAME: oracleservicesun
TYPE: 10 WIN32_OWN_PROCESS
STATE: 4 RUNNING
(STOPPABLE, PAUSABLE, ACCEPTS_SHUTDOWN)
WIN32_EXIT_CODE: 0 (0x0)
SERVICE_EXIT_CODE: 0 (0x0)
CHECKPOINT: 0x0
WAIT_HINT: 0x0

2. Use the Oracle 10.2 server software to connect to the database normally, as follows: C: \ Windows \ system32> set ORACLE_SID = sun
C: \ Windows \ system32> e: \ oracle \ product \ 10.2.0 \ db_1 \ BIN \ sqlplus.exe/as sysdba
SQL * Plus: Release 10.2.0.4.0-Production on Monday May 14 17:06:32 2012
Copyright (c) 1982,200 7, Oracle. All Rights Reserved.

Connect:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0-64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

Sys @ SUN>

3. Use the Oracle 11.2 server software to connect to the database normally, as follows: C: \ Windows \ system32> set ORACLE_SID = sun
C: \ Windows \ system32> e: \ oracle \ product \ 11.2.0 \ db_1 \ BIN \ sqlplus.exe/as sysdba
SQL * Plus: Release 11.2.0.1.0Production on Monday May 14 17:07:23 2012
Copyright (c) 1982,201 0, Oracle. All rights reserved.

Connect:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0-64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

Sys @ SUN>

4. Use the Oracle 10.2 client to connect to the database as follows:
C: \ Windows \ system32> set ORACLE_SID = sun
C: \ Windows \ system32> E: \ oracle \ product \ 10.2.0 \ client_1 \ BIN \ sqlplus.exe/as sysdba
SQL * Plus: Release 10.2.0.4.0-Production on Monday May 14 17:11:27 2012
Copyright (c) 1982,200 7, Oracle. All Rights Reserved.

Connect:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0-64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

Sys @ SUN>

5. You cannot connect to the database using the Oracle 11.2 client, as shown below:
C: \ Windows \ system32> set ORACLE_SID = sun
C: \ Windows \ system32> e: \ oracle \ product \ 11.2.0 \ client_1 \ BIN \ sqlplus.exe/as sysdba
SQL * Plus: Release 11.2.0.1.0 Production on Monday May 14 17:13:08 2012
Copyright (c) 1982,201 0, Oracle. All rights reserved.

ERROR:
ORA-12560: TNS: protocol adapter error.

Enter the User Name:

Additional information:
The sqlnet. ora file of the Oracle 11.2 client is as follows:
# Sqlnet. ora Network Configuration File: e: \ oracle \ product \ 11.2.0 \ client_1 \ network \ admin \ sqlnet. ora
# Generated by Oracle configuration tools.

# This file is actually generated by netca. But if MERs choose
# Install "Software Only", this file wont exist and without the native
# Authentication, they will not be able to connect to the database on NT.

SQLNET. AUTHENTICATION_SERVICES = (CNT)
NAMES. DIRECTORY_PATH = (TNSNAMES, EZCONNECT)
Please give me some advice, and I haven't figured out the cause for a long time ..