Help: Dumb newbie locked himself out!

Hello all,

sorry for the dumb question but I managed to mess up the privileges in my
database and now I'm unable to create or modify any users. I can still
connect to my database using the postgres user but I don't have the
permissions to do anything. How can I reset the permissions so that the
postgres user will have full access again?

This is a RedHat Linux 7.2 machine running PostgreSQL 7.1.3, standard RPM
installation of everything.

Thank you for you help in advance,

Balazs