SQL & PL/SQL

I have a prolem with External proceduere uses C as language

i placed the dll file in oracle_home/bin directory

and run the procedure, then i got an error which is given below

SQL> exec shell('dir')
BEGIN shell('dir'); END;

*
ERROR at line 1:
ORA-28595: Extproc agent : Invalid DLL Path
ORA-06512: at "ENCORA.SHELL", line 0
ORA-06512: at line 1

Then,

i configured the listener.ora with (ENVS="EXTPROC_DLLS=ANY")
then i got the following error
.SQL> exec shell('dir')
BEGIN shell('dir'); END;

*
ERROR at line 1:
ORA-06520: PL/SQL: Error loading external library
ORA-06522: Unable to load DLL
ORA-06512: at "ENCORA.SHELL", line 0
ORA-06512: at line 1


if,i tried with out setting (ENVS="EXTPROC_DLLS=ANY") then got Extproc agent : Invalid DLL Path.
if, i set the environment variable , then got unable to load the external library.

can any one suggest me how can i solve this?