Gods, don't be so melodramatic and naive. These days Usenet is full of posts, and the pornographers are posting to everything from alt.comp.literacy to alt.binaries.pictures.landscapes and everything in between. Perhaps even the FBI is posting it as well, who knows.
To preserve privacy on your machine, for whatever reason, it's as simple as:
a) Use TOR! Preserver your privacy online by using onion routing networks. It's good sense for web browsing.
a) Use the built in privacy features in your browser to delete the cache when you finish browsing.
b) Use any of a number of utilities to overwrite the free space of your drive with zeroes. Heck, it's built in with mac OS X
--no-- computer forensic techiques exist to recover data once it has been overwritten once with zeros. There is a long standing challenge for any data recovery company to retrieve data from a hard drive that has been "dd if=/dev/zero of=/dev/whatever"ed to death. They won't take it, and for good reason.
That's what it boils down to. You don't need to smash your hard disk. You do need to be a levelheaded, smart person.