CiviCRM on Drupal begins Install and then reverts to "Page not Found"
Check to be absolutely sure you've got CiviCRM installed to sites/all/modules/civicrm.

Also check that you aren't getting bounced from host A to host B, or from http to https, during the installation.

If that doesn't work, watch the PHP error log and webserver access log during the installation; that may show you what's not otherwise obvious.