I personally use Killdisk, it's a free program that does just that. You can go on their website, download and burn a bootable CD with it (website gives all details), then boot your system from that CD and proceed according to the screen instructions. on the left menu pick one-pass erase, which is the free method, make sure you select ONLY the HDD or partition you want to do this on , then proceed.
It will write only zeros on the whole partition, rendering it in a clean, factory-like state.
Depending on the size of the HDD or partition, it may take a while. When done, exit, remove CD, reboot, install OS of choice.