problem in logging into database

I am new to using postgres. I working on version 8.0.

I have created a number of users in the database but I am not able to log
into it using any of them, where in logging into using user postgres is
successfull .... the commands that i am using are

-- createuser -a -d -P "username"

for logging into , i am using
-- psql -U "username"

In all the references online, it says that the process shud work.
The postmaster service and databases are running.

=====-----=====-----=====
Notice: The information contained in this e-mail
message and/or attachments to it may contain
confidential or privileged information. If you are
not the intended recipient, any dissemination, use,
review, distribution, printing or copying of the
information contained in this e-mail message
and/or attachments to it are strictly prohibited. If
you have received this communication in error,
please notify us by reply e-mail or telephone and
immediately and permanently delete the message
and any attachments. Thank you