Completely erasing a HDD?

I'm about to replace Windows, so I'm taking the chance to completely erase my HDD. I've been letting junk accumulate for a few years.

I'm not sure how to completely erase a hard drive though. I've been told that formatting it doesn't actually delete stuff, and therefore wouldn't recover all the disk space? I'm not trying to hide anything, so I don't need something for that. I just want a fresh start for the new install of Windows.

What's the best way to erase a hard drive?
 
Solution
Formatting should do. It doesn't delete anything but it makes the drive forget that the data is there, therefore recovering the space. The data will be completely removed when it's overwritten.