Security!!!

I wonder if my setting have problem or not. I found that there is no
security issue on my postgres95.

1. I have a super user postgres, however, when I login as somebody else
which should not have right to the postgres95 database, I can use the
following command to act as postgres super user.

psql -u "database name"
>>Username : postgres
>>password : "anythingelse"

After this command, I can use the sql monitor as super user postgres,
which means that I can create, drop, update anything or destroy everything
on the database only if I can login to the server which have postgres95
installed.

2. I am using Postodbc as odbc driver on windows 95. Same as above, I can
login as anybody else without a password check even I use "alter user
[username] with password [password]" to add a password.

Walking through the documents, still didn't find any solution.
Please help if there is a solution to this problem. THx
(Linux 2.0.30 (standard), postgres 6.3, gcc 2.7, windows 95 OSR2)