Check that ORACLE_HOME path specification in your listener.ora is correct. Something like in this example:
Code:
....
    (SID_DESC =
      (GLOBAL_DBNAME = O101)
      (ORACLE_HOME = C:\oracle\product\10.1.0\Db_1)
      (SID_NAME = O101)
    )
....
Better yet, post your listener.ora and sqlnet.ora.