in fact, in my opinion, even if Microsoft Word is intelligent enough to correct all mistakes, we should refrain from using it, especially for people who wish to improve grammar and spelling.
Why I say so? Because Word changes the mistakes automatically for you, as you type. You may not even be aware of those mistakes and they are already corrected by Word. How then can you know your problems and avoid them in future?
My suggestion is to use Notepad to type your compostion. After you finish, copy and paste it into Word and let Word do proof-reading for you. You can see your mistakes immediately.
Of course, like what Arael said, Word is simply inept in correcting all the mistakes, especially those less obvious ones. And sometimes it changes what is actually correct. I'm used to British spelling. Everytime I type 'ise', Word changes it to 'ize'. I set the default dictionary to be British, but it stubbornly changes back to American by itself after some time. It's hopeless.