I have used ViaVoice (cheap) and Dragon Naturally Speaking (expensive). It appears "you get what you pay for" is true in this case. It took 6 months to get ViaVoice to a 80% recognition - it took 6 weeks to get Dragon to a 98% recognition. Obviously, I recommend Dragon. Just be prepared that it takes a pretty serious commitment to "train" it to each individual voice.

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Lily Chambers, The Virtual Office Goddess, LLC
Author "How to Do Your Own Small Business Bookkeeping"