one cause I believe is the gouvernment but more precisly the riaa that drops files on the network here are some ip's to block as most crap comes from them
192.168.*.*
10.*.*.*

the rest comes from people who download but don't check if the file is corrupt or not. I always do as soon as there's a new file in my downloads folder I check it. we should all. but of course some might not read this or do what is said here: check for corrupt files. we need to stop the spreading of these bad files