I can think only a few possibilities, given that you're software and OS are all patched properly.
Disclaimer: I don't use Outlook Express, and I first started using Office with the XP version. So I know almost nothing specific about the two pieces of software we're discussing. Nice, eh?
You said that you you went into Tools | Options in Outlook Express and "changed it to English", meaning the language selection. Do you mean that it actually indicated another language (French, for instance) initially in the settings? If so, I'd have to wonder what caused that indication. If you patched IE (and thereby OE) to SP1 (the browser SP1, not the OS SP1) then are you certain that you didn't apply a French version of the patch? When you go into an Office app (Word might be best for our purposes.) and look under Tools | Languages | Set Language what setting is listed there? (I'm guessing that the Office 2000 menu choices are similar to those in Office XP.) If it isn't English, it should be. I'm trying to figure out how Outlook Express got the idea that it should be using the French dictionary for spell checking. And I'm not even sure about the first possibility I mentioned since, what with the implementation of Unicode and all of this multi-language capability in this OS, I'm not sure there IS actually a French version of the SP1 patch. BTW, while you're in that Tools | Language | Set Language dialog, is the "Detect language automatically" selection checkmarked? If so, you might try unchecking it to see if that makes a difference. I'm thinking that, maybe, it's possible for some specific content that you use in your documents (a signature, perhaps?) that is fooling the spell checker into thinking that it should be using the French dictionary.
A final note. If you've changed the dictionary language and are trying to check documents that were created BEFORE you changed the dictionary choice, my guess is that you're out of luck. I think the change will only apply to documents created AFTER the change was made. This would be due to the way the documents are constructed by Office so that they can be checked. I could be wrong about that, since it is only a surmise based upon limited observation, and NOT upon some super secret knowledge of the inner workings of Office apps.
Good luck!
- prosaic