The tower should be fairly easy to get into, simply by undoing a few screws. (I'm glad that you didn't say that it's a Dell computer; they can be a nightmare to open up!).
The hard drive is easy to spot. It's just a metal box with a ribbon cable and a power cable attached to it. There are two of them in the cradle in this picture:
http://everythingaboutideharddisk.com/wp-content/uploads/2007/11/everythingharddisk-post-2.jpg
(Getting a hard drive out is usually very simple. Just unplug the cables and remove the small screws which hold it in its cradle).
To destroy the hard drive, either bash it with a hammer or (as I do) prize the metal case open and empty a tube of superglue into it.