Are you using windows? Open the registry and go to HKEY_LOCAL_MACHINE/SOFTWARE/ORACLE/HOME0 and look at NLS_LANG and make sure that it, "AMERICAN_AMERICA.WE8ISO8859P1", matches up with the result of
Code:
SQL > select * 
from V$NLS_PARAMETERS 
where parameter='NLS_CHARACTERSET';

PARAMETER               VALUE
--------------------- -------------------
NLS_CHARACTERSET      WE8ISO8859P1