Have you removed all the windows accounts or forgetten the sa password?   You still have one more option.

Check out the new KB article, "How to make unwanted access to SQL Server 2005 by an operating system administrator more difficult".

In it, you'll see the following interesting blurb:

"...if SQL Server 2005 is started in single-user mode, any user who has membership in the BUILTIN\Administrators group can connect to SQL Server 2005 as a SQL Server administrator. The user can connect regardless of whether the BUILTIN\Administrators group has been granted a server login that is provisioned in the SYSADMIN fixed server role. This behavior is by design. This behavior is intended to be used for data recovery scenarios."

So if you have access as an administrator to the machine, you can still access SQL Server - but single-user mode will be required.