I have had the Yahoo toolbar download onto my computer without knowing it several times until it appeared and frustrated me to no end. I had a hard time using Google because Yahoo would take over.
I do/did two things to free myself from this intrusive menace.
(1) ANY TIME YOU DOWNLOAD ANYTHING, WATCH CAREFULLY FOR THE DETAILS IN WHAT YOU ACCEPT, CHECK THE BOXES FOR, OR WHATEVER YOU DO. This toolbar comes included with many programs and add ons, and as you go through the process of downloading, it is easy to click the "ok" to download or a button to "accept" the Yahoo Toolbar, in error, Never realizing what you've done it until you're staring at it with your eyes crossed, trying to figure out how to get rid of it.
(2) If I found I had it on my PC I would remove it through Add/Remove Programs, find it and remove it. However, I think they must have caught on to the fact that people who use computers aren't dumb and lately I've found it can be harder to find in the Add/Remove Program. In that case I go to "View" and check the box that allows you to choose what toolbars you want to use. I just make sure it is checked/not checked indicating that it should not show at all, as if it isn't there at all. It's still downloaded but it won't "make you tear your hair out over dealing with it.
I am now using a Mac Book Air and haven't yet had the problem, therefore I have no idea what one would do if it was downloaded onto a Mac except to be vigilant and know what boxes you are checking (ticking) to avoid downloading it from the start.
Hope this makes sense and didn't confuse anyone.