1st.the RIAA does random searches on random pcs, and checks, if you brag about downloading things like that publicly, they will probably just trace you IP address through, and check through that way.
2nd.Thats called hacking unless you didnt know, and if the RIAA didnt know that somebody else had gained unauthorised access to the network, then yes they would be fined/imprisioned.
Hope that cleared things up!
P.S If you are one these people who downloads stuff (mp3s,avis)
then get a firewall, or there is a program that blocks out certain IP address, trust me we all need it!