I actually noticed this kind of thing in a couple of ways: 1. after I downloaded the FC4 DVD .iso, several other files on that PC became infected with a generic backdoor trojan, & 2. browsing around PirateBay, I found innumerable files with names that made them look legit, but were actually just spyware. Be careful which torrents you hit...

"Boyd, widely known for chronicling spyware, hacking and malware exploits, has published details of the BitTorrent distributions and identified Direct Revenue and Marketing Metrix Group as the companies responsible for the rigged files."