Why does Mail.app translate the :-) smiley into a computer icon?
For whatever reason, the people sending you emails are sending the smileys in Wingdings.  With the normal Microsoft encoding of Wingdings, the letters and punctuation map to the symbols. 

Presumably, you have MS Office for Mac installed, because that installs a version of the font which includes these symbols in the normal character slots.  That's why you get the funny symbols.

By default, Mac OS X ships with a version of the font that does NOT have those symbols assigned to letters and punctuation.  In that case, apps will display normal letters.  

The best solution would be to stop the sender from setting weird wingdings font on :-), but that may not be possible.

Another option is to remove the Microsoft version of the font and leave the OS X version. But that would mean that if you try to use Wingdings characters by typing letters, it wouldn't work. You'd need to add them from the character panel.