How To Delete Everything Off a Hard Drive?

I have a normal laptop pc with a 160 GB hard drive, it's a couple of years old and there's over 40GB of useless crap in there. The laptop is extremely slow (it didn't use to be) and I was wondering if there was a program where it could erase every god damn thing in there EXCEPT THE OPERATING SYSTEM. I heard that formatting doesnt do anything but remove the short cuts to the files, if you are going to suggest a program then please make sure its freeware because I'm a little short on money and I'm not the kind of person that buys stuff online (never have and never will). Thanks guys and sorry if i sound demanding (I'm a college student and the laptop is kind of essential for my studies. CHeers and all opinions welcome :-)