SQL & PL/SQL

Hi,
I've problems in creating spool file. I've some PL-SQL scripts, running monthly with sqlplus on Windows 2000, that have always generated spool file using the statements "spool filename.log".
Two days ago, I run this scripts and I noticed that they didn't create the log file, but nothing has been changed.

I noticed that changing the script with "spool c:\filename.log" the spool file is correctly generated.

I run the script using sqlplus user/pwd@db @c:\script.sql and in the body of the script there is:
whenever oserror exit SQL.OSCODE rollback
whenever sqlerror exit SQL.SQLCODE rollback

spool faXXXXX_X.log

Set autocommit off timing on echo on feedback on termout on serveroutput on

set escape ^

DECLARE
...

Why "spool filename.log" suddenly stop to work? There is some enviroment variable who determine the default directory of spooled file?