http://www.networkworld.com/news/2010/121110-google-microsoft-ad-networks-briefly.html

<<< For a brief period this week, cybercriminals managed to infect Google's and Microsoft's online ad networks with malicious advertisements that attacked users' PCs, according to security consultancy Armorize.

The attacks started around Dec. 5 and lasted a few days, sending victims who clicked on the ads to malicious Web pages... >>>