Google has started alerting users running Windows about a strain of malware that affects the Google search results. The malware infiltrates PCs using scams offering antivirus software via download.

The warning appears as a bright yellow banner at the top of the page after users conduct a search with Google that reads “It appears that your computer is infected with software that intercepts your connection to Google and other sites. Learn how to fix this.”

The malware prompts infected Windows computers to send traffic to Google through proxy servers thereby hijacking the search results for payment. Google is detecting traffic that comes from these servers and notifying users sending the traffic that their computers appear to be infected. Google first posted the warning on Tuesday after it detected what it called “unusual search traffic” when doing maintenance at one of its data centers. Google decided that the abnormal traffic was a symptom of infected PCs.

Google says that that several million PCs appear to be affected, that it has warned several hundred thousand people, and that the source of the infection appears to be one of roughly a hundred variants of fake antivirus software. The company says it is not aware of a specific name for the fake antivirus software responsible for the infection.

Google can only warn the users that their machines are infected. People must run a legitimate antivirus program to detect and delete the threats.