Google is issuing warnings to people whose computers are infected with a type of malware that manipulates search requests.
A strain of rogue anti-virus software also includes a search hijacker component. The hijacker is designed to further enrich scammers by redirecting users of compromised machines through various dodgy pay-per-click affiliate sites instead of genuine search engines.
Instead of going straight to Google, surfers on compromised machines are sent via proxies. The traffic generated from malware-infected machines has an unique signature that has allowed Google to return warnings to victims using these machines.