It's mostly spam, viruses put on the network to discourage people from using p2p. Probably put there by the riaa, mpaa, and others trying to promote their sites.
This sticky has links to a lot of info on the subject:
Major Topics for Upload/Download Help
Also LW has a junk filter and you can train it to filter out most of the offending files, and you can add words to the keyword filter that you don't want showing up in your search results.
For info on viruses go here:
WARNING: Viruses on network you should be aware of!