DragonSpeak is good. However, Vista has voice recognition built in that works brilliantly with Microsoft Word -- as long as you speak clearly, it will recognize natural speech, i.e. just dictate, no need to go word-at-a-time.

I wouldn't necessarily recommend getting Vista just for this feature, but if you already have it, it works great.

Note, too, that which ever solution you use, a good microphone and quiet environment is critical.