Seems some adwords advertisers wanted a bit more out of visitors than visitors may have wanted to give:
Â ”Google has removed paid links that advertised seemingly legitimate Web sites but actually tried to install nefarious programs on PCs. The links were displayed as “sponsored links” after visitors entered specific queries into Google’s search service. Clicking the links would ultimately go to a legitimate site, but by way of another site that attempted a “drive-by installation” of password-stealing software. Miscreants placed the links using Google’s AdWords service for advertisers. “Google identified and canceled AdWords accounts displaying ads that re-directed users to malicious sites,” a company representative wrote on a corporate blog“.