Louis
You need to ensure that you have something set for the first two arguments of SetActiveSpellChecking. So assuming that you have an editor control on a form and a c1 spell checker and they are using the default names assigned to them when you drag the controls onto your form the following should work:
C1SpellChecker1.SetActiveSpellChecking(C1Editor1,C1Editor1.GetActiveXInstance(),False)
Dom