Hey there, nice question indeed,
Anyways, frankly, these kinda symbols are commonly used in MSN or yahoo or anywhere
Like two people talking,
Hey where have u been,
Oh, I was away XD or : D
Ok, let’s back to ur inquiry, this XD represents hilarious faces which are used
, In MSN or yahoo or anywhere else, they look like those

and there are some more shortcuts like
: ) or : ) ;) :P
some will come like
;).. or

and if u open up the MSN u will see them
i hope u got the point...
c ya