A detailed AV-TEST study recently revealed that although search engine operators such as Google and Bing make a lot of effort to avoid doing so, they sometimes deliver websites infected with Trojans and similar malware among their top search results. Other search engines do an even worse job.
AV-TEST took 18 months to carry out a study that would confirm its initial suspicion, namely that the best-known search engines Google and Bing are being used to include websites containing malware among the top search results delivered to users. This exploitation of search engines is causing their operators to be tripped up by their very own systems. During its study, the institute investigated over 40 million websites delivered as search engine results.
AV-TEST report (PDF)