Is your computer free of child porn? Are you sure? While trojans can silently plant child porn in your computer's disk drives, there are far easier paths. For example, they can get there through RSS feeds, pulled by your favorite news aggregator. Aggregation feeds are particularly risky.
Shocking? Not really. The danger has always been there. Child porn images may be lurking within any of the web pages out there. There is no easy way to detect unasked for resources being downloaded silently and invisibly while you are looking at a page. They usually end up in your browser cache. Even after the cache is deleted, bits of the images are likely to remain on your disk ready to be used as forensic evidence against you.
What else can they place on your disk drive that can get you into trouble? Government or industrial secrets for sure. File this thought under What You See Is Only Part Of What You Got (WYSIOPOWYG?).