Question on debugging TNS-12560

Hello all, 

I have oracle 9i installed on win2k. It works without problem 
most of the time, but once in a while I get TNS-12560:protocol 
adapter error --> during login <--. 

I know re-installation is probably one solution which I did 
earlier but seeing the mess it involves, I am not very inclined 
to do so. 

Following the suggestions of an earlier post in this newsgroup, 
I tried the following things in a DOS window:

1. Ping my machine: Worked
2. tnsping my ORACLE_SID (which is ORACL): Gave OK.
3. set ORACLE_SID=ORACL 
4. sqlplus system/manger: This still gave a 12560 error.

I am a novice in this. Can somebody please let me know 
how to debug this problem? I am attaching my listener.ora 
and tnsnames.ora from the D:\oracle\ora92\network\admin 
directory if that helps.

Regards,
- Swapnajit.

listener.ora ++++++++++++++++++++++++++++++++++++++++++++
# LISTENER.ORA Network Configuration File:
D:\oracle\ora92\network\admin\listener.ora
# Generated by Oracle configuration tools.

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC0))
      )
      (ADDRESS_LIST =
        (ADDRESS = (PROTOCOL = TCP)(HOST = PC-MITTRA)(PORT = 1521))
      )
    )
  )

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PLSExtProc)
      (ORACLE_HOME = D:\oracle\ora92)
      (PROGRAM = extproc)
    )
    (SID_DESC =
      (GLOBAL_DBNAME = oracl.localhost)
      (ORACLE_HOME = D:\oracle\ora92)
      (SID_NAME = oracl)
    )
  )

tnsnames.ora +++++++++++++++++++++++++++++++++++++++++++++++++++++++
# TNSNAMES.ORA Network Configuration File:
D:\oracle\ora92\network\admin\tnsnames.ora
# Generated by Oracle configuration tools.

ORACL =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = PC-MITTRA)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = oracl.localhost)
    )
  )

INST1_HTTP =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = PC-MITTRA)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SERVER = SHARED)
      (SERVICE_NAME = MODOSE)
      (PRESENTATION = http://HRService)
    )
  )

EXTPROC_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC0))
    )
    (CONNECT_DATA =
      (SID = PLSExtProc)
      (PRESENTATION = RO)
    )
  )