As I posted this morning (
The post was deleted by mistake, right? 
),
There's no point in contacting the authorities now because you have no evidence to
offer them and they cannot do anything. In fact, since you deleted it they just
might decide to seize your computer anyway just to cover their own sorry asses.
P2P is not a good source for tips anyway. The police pretty much have to patrol P2P
directly because then they can save evidence [such as IP addresses plus hash plus
exact time all together] as they find them and can then get warrants right away.
Working from a tip, they would have no choice but to go to the P2P network and
retrace your steps in hopes that they may find the bad file again. Since a tip is just
hearsay, they cannot go directly to a warrant and the chaotic nature of gnutella
means they might not be able to find it even if your tip is detailed and accurate.
As for wiping away traces of that one you almost downloaded, a defrag will be
good enough because your disk is already carrying a lot of data and defragmenting
is likely to wipe all the free sectors which once held the recently deleted file. You
should also be able to wipe free space with that nuker program, if it's anything like
the other good erasers you can get for free. That should be much more than
enough security for you.