It may ahve been installed at the same time you installed The toolbar if you do have that. But I know it doesnt just show up… if it did not sure how..

anyway that is neither here not there..

go to control panel
add/remove Yahoo Courier

and hope that will work for you.

There are additional programs to help remove programs you dont want really
You need to make sure that it is out of registry too
I use East-Tec eraser and it really removes the program (to my information)