If infections that OTHER computers have caught counts... like some others here, I deal with fake AVs and browser-addon adware all the time (easily half of the jobs I get), but a few months ago I ran across this one Vista laptop that caught an infection that actually unregistered, modified the filesystem flags on, and then deleted the DLL files responsible for the windows update service, update checking/logging, and BITS... must have taken me a good two to three hours to figure out and fix the issues, and of course it was like 3 years behind on updates by the time I got to it.