Thank ya, thank ya sir. Just so i'm clear. Writing 0' s on a disk kinda resets it to a brand new hard drive, in a matter of speaking? And restoring the drive to factory kinda overwrites it with orginal microsoft data?
Like Dauntae said, when you delete a file it isn't actually deleted until it's overwritten with other data (hence the reason why basic software like Recuva works in recovering freshly-deleted files). When you click "delete" on a file it just marks the sector(s) that file takes up as writable. So the next time a file is saved, those sectors are available for writing.
So, writing 0's essentially breaks all the links to the files, and then saves data on top of those sectors so that the data is not recoverable. If you do a factory reset, like from HP or Dell, all that does is run a quick format on the drive (break all the links), and reinstall the OS, which will potentially overwrite some of the previously saved data.