Install password not recognized

Hello,  I just installed 8.1.3, for the first time on an XP machine, and
immediately had a superuser password problem, so decided to uninstall
and reinstall.  However now, with the reinstall, I can't get access to
the db.  pgadminIII superuser password request returns the fatal error:
" password for authenticataion failed for user postgres" message.  Also
neither the original password or my 'new' password works with psql
command line prompt. So I seem to be stuck... and can't get into the
database.

Can anyone say how to properly reset the authentication password, or
baring that, how to kill the residual password file lurking about on my
machine, so I can do a fresh reinstall?

Thanks much.

Glenn Brooks