According to Web Hosting Talk

A recent survey by Google's Anti-Malware Team seems to confirm what many of us have believed in the past: Microsoft's IIS (Internet Information Services) servers are more vulnerable to host malware than Apache servers. The statistics come from the recently launched Google Online Security Blog whereby Google's researchers looked at 70,000 domains that were either distributing malware or hosting attack code.

Google however does make it clear in its blog that just because IIS is serving malware doesn't mean that it has been compromised; it might be possible that most 'warez' related Web sites uses IIS. This study has caused many heated discussions amongst security advocates and bloggers who claim that this might be an emerging type of gamesmanship from Google to promote Apache over another product made by its rival.