
Even if they find your track(s) on a P2P network, they still have to prove that you put them there. OK, in most cases that would be trivial (e.g. your IP address is found as the source, and/or they seize your computer and find the P2P set up, etc) but the release of such a file is not, in itself, enough.
What if it your PC was not properly protected and so compromised by a virus that allowed others access to your files? What if you changed HDD and did not properly erase/wipe it?
Yes, I know those are really DUMB things to allow considering what it could reveal about your personal life & bank accounts, but it still happens in lots of high and low places.
What if you lost a USB stick or MP3 player with your collection on it?
They need to actually bring a proper prosecution, although I suspect a lot of bully-like legal FUD would be used to convince you otherwise, and you would have to be able to provide a plausible denial that would convince both an expert witness and jury.
A properly wiped and re-installed disk might cover most of your tracks if you have the time to react (i.e. not finding out when the police are at your door with a warrant), but evidence of P2P use from your ISP may be there, and you may need to come up with a very good reason for 'destroying evidence' in performing the wipe. Enough for you to stand up to under cross-examination?