A website about drugs (legal OTC and prescription stuff) that was the top result on Google recently downloaded a trojan on my computer. The stupid thing would show as infecting my computer everytime I rebooted even after the anti-virus said it cleaned it. It took several hours of running several online AV scans, deleting, rebooting and deleting system restores to get the stupid thing clean.
I've used them both for a while and very rarely do I see an advertisement, anymore.