postgresql startup

I am using Ubuntu 7.10. When trying to use Postgresql 8.2 I get the
following errors:

root@DagonUX32:~# su -c postgres
bash: postgres: command not found
root@DagonUX32:~# createuser -s -P dagon
Enter password for new role:
Enter it again:
createuser: could not connect to database postgres: FATAL:  role "root"
does not exist

I have done a complete removal of postgresql, then resinstalled. I still
get the same error. Is there a configuration file somewhere that needs
to be removed? (I did make sure that /etc/postgresql was deleted.

All the help I can get will be appreciated!

Thanks.

Charles