Problems using PostgreSQL command line tools

Hi to all in the mailing-list! I am new and I have problems with command
line tools of PostgreSQL.

I have 2 computers, one at home and one at office. Both computers are
equipped with Windows 2000 Professional SP4. On both computers I have
installed PostgreSQL 8.1.3-1 (the latest at this moment).
Both installations are "clean", I have NOT modified any PostgreSQL
configuration file!

At home (where I have no problems), I can do:
-----------from console-----------
C:\>psql -h localhost -U postgres -d postgres
Password for user postgres:
Welcome to psql 8.1.3, the PostgreSQL interactive terminal.

[....]

postgres=#
----------------------------------

At office, if I try to do the same thing, I get:
-----------from console-----------
C:\>psql -h localhost -U postgres -d postgres
psql: FATAL: password authentication failed for user "postgres"
----------------------------------

In practice, psql doesn't ask me the password and fails immediately!
But note this (the first strange thing), I have NO problems connecting
to PostgreSQL server using pgAdmin III tool and also from PHP4/PHP5
scripts (I develop on web with Apache and PHP4/5).

For example I have tried the following PHP script:
-----------code-----------
<h1>PostgreSQL connection test</h1>
<?php
$dbconn = pg_connect ("host=localhost dbname=postgres user=postgres
password=pgpw")
or die ('Connect error: ' . pg_last_error ());

pg_close ($dbconn);
?>
<h3>Connected!</h3>
--------------------------
And in the web browser I see "Connected!", so connection to PostgreSQL
is ok!

I am NOT able to connect to server using all command line tools like
psql, createuser, createdb, etc...

Can someone help me?? What could be wrong on my computer at office???
Thanks in advance and sorry if my english is not perfect!

Andrea