This is important: Google has been able to detect a large number of computers infected with a specific piece of malware. If you go to Google and do a search (any word will do) right now, check to see whether you get a "Your computer appears to be infected" warning at the top of the search results. If you see the message, you need to clean up the infection from your machine.
We're trying this as an experiment to alert and protect consumers that we believe have infected machines. Please share this widely.
Added: This is malware that's specific to Windows. Remember to do an actual search (any search will do) and check the top of the search results page; don't just go to the home page.