Experienced the same problem, and changed the file joomla.sql in installation/sql/mysql to read
Use at your own risk - I am not a skilled user, but it worked for me. I re-ran the installation procedure (it had paused at this error).
--
-- Table structure for table `jos_usergroups`
--
CREATE TABLE IF NOT EXISTS `jos_usergroups` (
`id` int(10) unsigned NOT NULL auto_increment COMMENT 'Primary Key',
`parent_id` int(10) unsigned NOT NULL default '0' COMMENT 'Adjacency List Reference Id',
`lft` int(11) NOT NULL default '0' COMMENT 'Nested set lft.',
`rgt` int(11) NOT NULL default '0' COMMENT 'Nested set rgt.',
`title` varchar(100) NOT NULL default '',
PRIMARY KEY (`id`),
UNIQUE KEY `idx_usergroup_parent_title_lookup` (`parent_id`,`title`),
KEY `idx_usergroup_title_lookup` (`title`),
KEY `idx_usergroup_adjacency_lookup` (`parent_id`),
KEY `idx_usergroup_nested_set_lookup` USING BTREE (`lft`,`rgt`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=9 ;
--
-- Dumping data for table `jos_usergroups`
--
INSERT INTO `jos_usergroups` (`id`, `parent_id`, `lft`, `rgt`, `title`) VALUES
(1, 0, 1, 20, 'Public'),
(2, 1, 6, 17, 'Registered'),
(3, 2, 7, 14, 'Author'),
(4, 3, 8, 11, 'Editor'),
(5, 4, 9, 10, 'Publisher'),
(6, 1, 2, 5, 'Manager'),
(7, 6, 3, 4, 'Administrator'),
(8, 1, 18, 19, 'Super Users');