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Google explains: "this warning message appears with search results that we've identified as sites that may install malicious software on your computer. We want our users to feel safer when they search the web, and we're continuously working to identify such dangerous sites and increase protection for our users."
Unlike spam pages that try to deceive search engines and are penalized or removed from the index, these pages deserve their ranking, but users are strongly discouraged to visit them.
A study from McAfee SiteAdvisor conducted in December 2006 revealed that: "41.4% of [dangerous] sites exhibit poor e-mail practices, 24.5% contain risky downloads, 26.8% are scam sites, 32.3% link to other risky sites and 3.0% contain browser exploits." Also, 4.2% of Google's search results are dangerous sites.