I just followed an advertisement in my Yahoo! mail for "3 easy tips to keep your PC clean and quick" which was advertised as part of Yahoo! and was not delivered through an e-mail I received.
I should be a bit more clear. My e-mail inbox was empty; I keep my e-mail accounts clean on a regular basis. When Yahoo! mail is empty, the area where e-mails usually show up becomes blank and then is replaced with advertisements. I assume these ads allow Yahoo! to have a free e-mail option for users by helping pay for the servers. At any rate, it was one of these advertisements I opened.
I will not link to it here as it contains a link to software that is malicious. The software claims that there are viruses on the system and asks for $30 to remove them. Just wanted to provide a warning for those who use Yahoo! mail.