Granted you have administrative Windows privileges on the server, another option would be to start SQL Server in Single User Mode, using the Startup parameter "-m". Doing this, you can login using SQLCMD, create a new user and give it sysadmin privileges. Finally, you have to disable Single User Mode, login to SSMS using your new user, and go to Segurity/Logins and change "sa" user password.
You can check this post: http://v-consult.be/2011/05/26/recover-sa-password-microsoft-sql-server-2008-r2/