Interesting post from the Official Google blog site. They've noticed that a number of computers that connect to search are showing strange behaviour and they have worked out that some machines have been infected with specific types of malware. Consequently, if they identify a computer that's exhibiting this behaviour they're putting a message up on their screen 'Your computer appears to be infected'.
All that you need to do to check is to go to the Google site (I assume .com) and type in a search (any search) and look at the results. If you see a dirty great big yellow banner at the top of the screen, you're one of the infected. There's a link on how to fix it as well.