You might repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair), then checking whether the problem is resolved.



It's also possible you have a faulty third-party addin. If running Word in safe mode (i.e. hold down the Crtl key while starting Word) resolves the problem, a faulty third-party addin is the most likely cause. In that case, re-start Word, disable all third-party addins, then progressively re-enable them until the culprit can be identified by its problematic behaviour. Uninstall (and, perhaps, reinstall a compatible version of) the addin.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

The problem's gone when I run Word in safe mode!!

However I've run both Quick and Online repair, but the problem is still there. I've also disabled the only add-in I had enabled and I also removed GM's template add-in from the Start up folder, but none of it is solving the problem when running Word normally.

When I create a new document, it shows up correctly in Letter mode. But when I open an existing document, it shows up in Email Message format and I must manually change it into Letter