I gave in to a masochistic urge to poke through fanfics on ff.net. One of the pop-ups it loaded appears to have been... Well if not malicious, at least it was very unfriendly. It reset my homepage from blank to whatever it's stupid homepage is. Then it installed something -- I have no idea what! -- on my hard drive. The first I know if it was that it popped up a message telling me that the newly installed software won't take effect until I reboot, and asking me if I wanted to reboot my computer. Excuse me!?! I didn't tell it to install anything! It didn't ask, I didn't tell.
What I did do was install something that's supposed to get rid of spyware and pop-up programs. It hasn't caught them all. I still get pop-ups just about every time I load a page -- even pages that don't normally have pop-ups. Grr!
But really, how is any of that legal? I did not give it permission to mess with my system! I don't think some third-party software should be allowed to mess with my system without my permission.