Upgrade to 2.7 site directs to wp-admin/install.php

I backed up my database and followed the instructions to upgrade to 2.7. When I went to wp-admin/upgrade.php, the page said the database is up to date. When I tried to log in, it redirected to wp-admin/install.php. I tried that and it wiped out my database. I restored the database from the backup, double-checked the table prefix and the wp-options table and both look fine. I tried again. Back to wp-admin/install.php.

I’m using php 5.0.

I think I upgraded from 2.5 to 2.7. I looked for version but didn’t see it on the bottom of the page, and didn’t think to upgrade in steps. This is the third site I’ve upgraded to 2.7 and the first time I’ve had a problem.