Turning off Exchange

I have a dell 2k server running SBS. I have Exchange installed but I am not using it but I don’t want to delete it. How do I turn off Exchange if I am not using it? Also what would cause the server to get slower as the day goes on? I ran the event logs and the only thing I see unusal is something called online defragmentation that runs serveral times a day but I didn’t see where or how to stop it.