This section of the vSphere 5.1 document describes how SSO can alter the security for vCenter 5.1 and who has access to login:
What I would do is login to the Web Client using that id/password you mentioned "from the beginning of the install" (default ID I believe is admin@system-domain.) Then select Administration -> Sign-On and Discovery -> Configuration and see what Identity Sources you have listed and let us know. If you need to, try adding the "Local OS" identity source. This is also where you can add your domain as an identity source if it's not listed.
Also under Administration check Access -> SSO Users and Groups and see if any users are locked/disabled that you would expect to be unlocked/enabled.
Finally, check Administration -> Access -> Role Manager and see if a vCenter Server is listed. If so, you should be able to see who is mapped to the Admin role.
SSO FAQ:
Hope this helps. If you make progress based on this, let us know and hopefully we can get you up & running again!