The HR Manager has a great posting on what to do when employees aren't getting along. Her advice in a nutshell? Tell them to grow up! Except she says it in a much more professional, helpful, and encouraging manner than that. She uses a great analogy of shaking monkeys out of a tree.
I wish I had read this 5 years ago. It would have saved me a lot of stress dealing with monkeys that had no business in my tree! Check out her posting here.