Look over at snapfiles.com/freeware and you'll find several programs that supposedly wipe a drive according to DoD specifications. I'll tell you one thing though, some computers that the DoD donates/sells to schools and places don't come with a hard drive at all. Hard drives that store sensitive information are destroyed.
Keep in mind that it takes a lot of time and effort to dissect a hard drive; the NSA or FBI would much rather use that time and effort looking at a suspected terrorists hard drive, than some little kids who downloaded the latest eminem album.
In other words, don't be so worried about it. That is, unless you have several terrabytes of illegal material that you upload to other limewire users every month, as these are the people the RIAA want to nail more. Why would the RIAA waste time and effort busting someone who pirated two albums when they could bust someone who pirated half a terrabyte!
It's all about time and effort and were it's best spent.
EDIT: And yes, there are much better and safer places to get music from; Limewire is not one I would recommend. You will have to search for better places yourself if you choose to pursue copyright infringement.