I would also like to add that even while uninstalling toolbars or "coupon" applications, they sometimes trick you into downloading something else, not only while installing.
I was removing some things he did not need, and was asked "Would you like to remove all content from this program?", which, if you're like me, always press "No" without reading the details first can be a pain. Make sure to remove all content.
There was also one where a pop-up opened while uninstalling and asked to download something else (I can't remember what it exactly was), but even while uninstalling they were setting advertisements.
It's ridiculous, to me, that people (like my brother) would download these things without even knowing it. Please please pleeeaseee read before installing anything.