Conficker Eye Chart
If this works, it’s a very clever idea. Members of the Conficker Working Group have created an eye chart that will tell you at-a-glance whether a Windows-based computer is infected with the Conficker worm. Point your browser to the eye chart. 
If you see all of the images, then the machine is probably clean. If some of the images are missing, then the machine might be infected with Conficker or one of its variants.
How it Works
Conficker blocks access to certain sites, those that might be used to download antivirus software. If that list of sites is blocked on a computer, images coming from those sites won’t appear.
Clever? Effective?