A recent case in which a man was accused of possessing child pornography was dropped, as it was discovered during the investigation that his computer had a virus that downloaded the illegal images onto his computer.
Stories like this beg the question -- could this happen to you?
Answer from Larry Magid, internet software and safety extraordinaire: "It is indeed possible for malicious software to plant child pornography--or any other type of file, for that matter--on an innocent person's computer, but being possible doesn't mean it's likely. And forensics experts can detect intention."
Translation: Possible? Yes -- Likely? No way.
Still, there are things you can do to protect yourself (though experts stress not to lose sleep over this):
1. Clean out your cache and cookies early and often
2. Clean out your Temp file in your C Drive too
3. Pay attention to the sites you visit and try to go to only trusted sources
4. Note times when your computer is acting sluggish and try to determine if something is being downloaded during those times (use Ctrl-Alt-Delete to look at programs that are currently running and do some searches on them if you do not recognize what they are)
But again, although charges of child pornography are indeed serious, this is not something that should cause panic given its high unlikelihood.