Don't be afraid of a little fat. Good fats are beneficial and necessary (yes, I said necessary). Peanut butter isn't so bad--though you do want to stay away from sweetened stuff. Sugar is bad news--besides causing weight gain, it can cause a true chemical addiction. All-natural PB, the kind you have to stir up and refrigerate, is much better for you. If you're not allergic to nuts, almond butter is even better for you in terms of good fats (it is a little expensive, but that'll help keep you from gorging ;)).
As long as you don't go nuts and eat half the jar, your after-work snack is not a killer. The protein, fat, and fiber will help keep you satisfied until dinnertime. Keep it to a tablespoon or so and there's no guilt in your snack. :)