Mail not opening files or Preferences menu

My Mail app is not allowing me to open emails in a new window only read them in the preview pane on the right. So I thought a setting in Preferences had gone adrift. But I cant open preferences either using either the drop down menu or shortcut: Cmd,
Mail works ie I can send and receive though suspect its slow as I always receive things on my iphone first.

What's going on?

I'm running OSX 10.8.3 Mountain Lion on a 6 month old iMac.

I've just reinstalled the OS and its had no effect.

Any advice much appreciated. Thanks