You have two choices:

1. By downloading the Google toolbar for your browser you get an excellent toolbar with several functions besides great spell checking. It works with both IE and Firefox. This is my favorite.
2. For IE users you can also download the "IESpell" add-in spell checker. We've used this one for years with IE and KeepAndShare and it works very well

In both cases the add in spell checking works best with the latest versions of the IE and Firefox browsers. So we strongly recommend you upgrade to IE 7.0 or Firefox 2.0. You'll find their security improved, their user interface improved, and their printing support improved.