When you type ":-)" in Word, it automatically corrects it into a smiley (J), which is the letter J in the Wingdings font. If you look at the HTML, you'll see:
If you would rather that it use the unicode smiley emoticon instead, do this in Word:
1. Insert | Symbol
2. Type in 263A in the character code field
3. Click AutoCorrect
4. This will populate the bar in "replace foo with bar" with the unicode smiley, so type 🙂 in the 'foo' part and then click Replace and choose to overwrite the existing one
If you send it in an email or save it as HTML, you'll now see something like:
 Personally, I think the inclusion of the smiley in word is a pretty smart thing. I remember typing out letters to my friends in the late 80s, and I'd use the sideways smiley in my letter but then would spend two lines explaining that they had to "turn their head sideways" to see it.
 I also think it's really smart to have the insert \ symbol dialog have a direct entrypoint to autocorrect. Nice little touches like that really help make software great.