there are two main ways for you to consider doing this; whichever way you choose would depend on your target audience.
the first method is performed server-side, for which you would probably need to install some type of component or 3rd party service.
the second method can be performed client-side, and the particular example i'm thinking of (i'm sure there are other ways) involved instantiating a microsoft word object as an ActiveX control to do the spell checking for me. BUT, that requires your visitors to have Word 2000 installed, and their brwoser secutiry settings turned down. so if this is for an intranet or personal use, you could go with this option. google will be your best friend on this one...
good luck!