Slanderous letter to HR from my Ex-Boss. What should I do?

Talk to HR about it. Obviously they did not believe the lies as you are still there. It really should not be a problem for you. As to signing a statement not to sue that is not unusual. As to the raises issue again talk to your new boss and find out why. It might not have anything to do with the letter. It could just be the state of the economy or the business you are in. Really no need of a lawyer at this point as you have no damages. You still have a job and you do not know that the letter is the reason you are not getting raises.