Interesting...
Ok, so lets try this:

Go to http://localhost/phpMyAdmin. Login with root/password

Go to the database list on the left and click 'nidb'. It will show a list of tables. Find the 'users' table. It should list out the rows in the users table, if not, click 'Browse' at the top of the page. Do any records show up, especially one with a username of 'admin'?

I'm thinking you may have downloaded an old version of the installer that didn't automatically setup the admin user account.