Email me for FREE viruses writes "Just how many people would click an ad saying "Is your PC virus-free? Get it infected here!"? According to the security researcher who ran that very ad on Google for 6 months, 0.16% (409 of 259,723) would click on it. 98% of those people were running Windows. The Google Adwords campaign cost $23 in total, which works out to $0.06 per infection had the site actually been malicious."