A common tactic used by malware is to block the infected computer from connecting to the Web sites of antivirus and security companies. Such blocks are meant to prevent you and your antivirus program from getting help in removing the infection.
The Conficker worm and many other types of malware take this step, and one good thing that came out of all the hype and drama surrounding last week's April 1 doomsday for Conficker was this little gem from the Conficker Working Group, an industry coalition formed to fight the worm.
The group's "Conficker Eye Chart" pulls images from three sites that Conficker is known to block and displays them in a box. Below the box is a guide to interpreting how you see the images -- if they all show up you're in good shape, but if one or more doesn't display it could indicate a Conficker (or other malware) infection.
It's a smart and near-instantaneous test that couldn't be any easier, but keep in mind that if your computer uses a proxy server for Web traffic, which can be the case in some companies, you might be infected and still be able to see the images.