IslandHopper, there is one solution to your spell-checker problem, if you're willing to switch to another browser. Firefox, which is a free browser, offers an automatic spell check feature in their latest version.
http://www.mozilla.com/en-US/firefox/
The spell-checker works by underlining misspelled words with a dashed red line after you've typed them. It won't go so far as to autocorrect your words, or to suggest what word you might want to use in the place of the misspelled one. Personally, I like it, as it provides an unobtrusive hint that you might want to change a word, but still allows you to choose to deliberately misspell a word if you want to.
However, if you're unwilling to make a switch to a different browser, you can add a spell-checker plug-in for Internet Explorer. It's free, according to the website.
http://www.iespell.com/
Unfortunately, I can't tell from their website if it works in IE7, or only in IE6.
Of course, the ultimate and drastic solution is to compose your replies in a simple, lightweight word processor or word editor, and once it's done to your satisfaction, cut and paste it into the reply form in the forum. However, I'm sure this has occurred to you already. I still do that upon occasion, but most often when I really want to make sure that I've composed my thoughts, and that what I have to say makes sense. It is not a practical solution for firing off a quick response when the inspiration hits you.