Something more important than arithmatic this time.

My colleague is having a serious problem with her outlook. Some messages are stuck in the outbox and seem to be jamming up the entire programme. Nothing can be sent, nothing can be exported, the error message says "message interface error". While trying to move these messages to the outbox, they seemingly vanished entirely.

The contacts book is huge and valuable but cannot be moved or exported so a reinstall is an extremely last resort..

Any ideas?