It wasn't BT it was Windows Mail, you did something that I would have tried instinctively but forgot to say, you toggled the options ie 'show all' to something else and back to 'show all' forcing it to actively reset 'show all'. The problem could have been caused by your massive Inbox. It's working now which is the main thing.
As you have a backup routine for M Documents why not create a folder called 'Windows Mail' in My Documents and then make this folder the storage location for windows mail
http://www.tech-recipes.com/rx/2273/vista_move_windows_mail_storage_folder/
You will then back up your emails every time you backup 'My Documents'.
As you appear to have an unstable large inbox I would advise backing up using KLS first.
As you have a backup routine for M Documents why not create a folder called 'Windows Mail' in My Documents and then make this folder the storage location for windows mail
http://www.tech-recipes.com/rx/2273/vista_move_windows_mail_storage_folder/
You will then back up your emails every time you backup 'My Documents'.
As you appear to have an unstable large inbox I would advise backing up using KLS first.