PHP error message at step 2 of installation

Ihave uploaded all the files and i when i try to install i get to the second step after entering blog name and email. At the second step the following message comes up:
Fatal error: Call to undefined function username_exists() in /home/a8623738/public_html/wordpress-2.8.5/wordpress/wp-admin/includes/upgrade.php on line 59
And when i try again to install it tells me i have already installed though i have not been through the other steps so i have to wipe the database and try again. I have done this several times and am now uncertain of what to do? Help please , Thanks