Windows – How to wipe Windows from the command line

A fast format (sometimes called quick format) only recreates the necessary table structures for the filesystem. The files aren't actually deleted, but when the table structures are reset to it's initial state they will be overwritten in time. This is because the system marked those positions as 'writable'.

A normal format recreates the table structures and clears out every block on the hard drive.

You can manually set the blocks to zero if you want using a tool like 'dd' on Linux.

Boot from the Mac OS X Snow Leopard Install DVD, and from the "Utilities" menu, choose "Disk utility". From there you can reformat your hard disk. After doing so, quit Disk Utility and you can perform a clean install.

EDIT: Apparently there is also an "Erase and Install" option. (I haven't used this before myself). See more details here.

Here's an official apple support article about all the installation options available.

It occurred to me after posting this that, while your question clearly states you want to format your disk, I wanted to mention that this will erase all data on your disk, including applications and your data. So make a backup first. Alternatively, you could just perform an "Archive and Install" which will give you a clean OS, but leave your data and applications in tact.