Recover SA Password - PSTools - Not working
Here is a technet article that has the detailed steps. 

In a nutshell you need to start SQL server in single user mode and connect using sqlcmd to add your account. This should get you what you need as long as you have admin rights at the OS level. 

One note is that if the sqlagent is configured to start up automatically you should disable that before you start SQL server in single user mode so the connection isn't taken by the sqlagent.