It will erase everything, to the point that a clean install of an OS will only show the files you put on from that day on...
If you are worried about it not being completely wiped clean, you will need to run a good hard drive utility that will low-level format the drive. This essentially re-writes the platters over with a series of ones and zeros, so that it is nothing can be recognized as actual data. (as mentioned above)
If that does not ease your mind, physically destroy the hard drive. Smash it into hundreds of pieces, then burn the remains.
I doubt you are trying to be that discreet with the data on that drive though,..right?
<b><a href=\"http://www.switchphase.net\" target=\"_blank\">Computer and I.T. solutions for small business</a></b><br><b><a href=\"http://www.laptoprepairdirect.com\" target=\"_blank\">Laptop Repair Direct</a></b>