looking for third party spell checking + dictionary software

We're working on a software project in which we want to implement spell checking. We don't want to write our own spell checking software from scratch, and we definitely don't want to write our own dictionary from scratch. We'd like to know what our options are.

Can we buy a spell checking dll along with a dictionary from Microsoft? From some other software company? Is there any open source freeware we can download?

Also, we would like to know what kind of overhead would be involved in incorporating such a package into our project. We're working in Visual Studios 2008 (possibly upgrading to 2010). How easy would it be to integrate third party spell checking/dictionary? What kind of access would we have in Visual Studios into that third party software (i.e. how easily can we customize the software via function calls, deleting/adding components (e.g. words in the dictionary), deleting/adding event handlers to events, etc.?)?

We would like as wide a variety of options as possible, so please, the more suggestions you have, the better. Thanks very much.