Perhaps the "enchant" library is an option? It is contained in the core of PHP 5.3 from what I read here (it replaces the pspell_ * functions, it seems) and can rely on things like the aspell dictionary.
http://us3.php.net/manual/en/book.enchant.php
If I remember correctly, aspell is also available for windows (Pidgin can rely on it), but you still have to rely on its dictionaries. I would recommend doing something native rather than NIH'ing something together. :)
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