It isn't ticked Ethel.....and won't let me tick it. yet when I tried to download a new Yahoo toolbar, I was informed that I had one already.....my windows toolbar is still there.
if it wont work, but won't let you add it again then try..
start menu > settings > control panel select add remove programs and then see if yahoo toolbar is in there, if it is remove it, then download and see if it will install again.