When you try spell checking the first time since you got Vista installed......
After creating a new email, when I click on the Spell check icon I received the
following error:

"This language is no longer available for spell checking. Please select
another in the spelling options dialog"

I went to Tools > Options > Spelling (note: in WinMail main page, not the message box)

English was already chosen, so something was not working.
Reselecting English and applying the change will correct this.