XChat for Windows: Automatic Spelling
All you need to do is download Aspell and an English dictionary.
Both these items are available at the
Aspell web site. It also
requires a recent version of XChat, 2.4.4 or newer.
As you type, your text will be automatically spell checked and
misspelled words underlined.
If you right-click inside the input box, you'll get options to insert
a spelling suggestion, or even turn off the spell checker.