How to allow anonymous user register Civicrm event without create drupal account

Question

I'm using D7 CiviCRM 4.4.6.

I had a civicrm event with profile fields for user to fill in.

Anonymous users are allowed to register the event and create civicrm profile.

But there is a "Username" field, which force anonymous user to create a drupal user account when registering the event.

Is there anyway which anonymous user can register in my event, creating profile, without create a drupal user account?