As I have a fresh install of windows on my work laptop, I also have a fresh install of thunderbird. When I started using it, it was underlining everything I typed as if misspelled. It obviously wasn't initialised or didn't have a dictionary installed. I couldn't see anything under options, so I tried installing an extra language to see if that would kick it into life. First mistake, I accidentally installed the language pack into firefox instead of thunderbird - make sure to read the instructions before installing, which does warn you of this -but I hardly ever read instructions.. Anyway that didn't fix it.

What did get it working in the end was pressing the spellcheck button within a message compose window and choosing a language...