Please see MSDN article "Creating a Custom Spell-Checker with Word 2003 and Visual Basic .NET" last seen here:

http://www.msdn.microsoft.com/office/understanding/word
    /articles/default.aspx?pull=/library/en-us
    /odc_wd2003_ta/html/odc_wdspellchkr.asp

Pros:

• You don't have to pay more for a third-party spelling package.

• You don't have to wait for a user-friendly, Windows Forms version of an open source spelling package to get its ASP-centric act together.

• You can use leverage the investment made in custom dictionaries of MS Word.

• Nothing extra to install on machines running MS Office.

Cons:

• Loading an instance of MS-Word takes several seconds and upwards of 30MB of memory by default. It is best to open Word on a separate thread.

• You can't be sure which version of Office this stuff works on: either Office XP or Office System 2003. You have carry around PIMs (Primary Interop Assemblies) for both.