Might be worth repairing privileges. You can do this by using Disk Utility which is in the Utilities subfolder in the Applications folder. Once Disk Utility is open then select your Hard Drive and click "Repair Disk Permissions".
Apart from that I can only think of an Archive and Reinstall using your System Disks, but that is a bit drastic. The only other thought I have is to see whether you can download Mail from the Apple Website and reinstall.