go to 'My Computer', 'right click' on the hard drive you wish to erase, you'll see an option 'Format...'
Format will erase everything on that disk,
However, should someone strangely want to go through length to do so, with the right tools, data can be recovered from a standard 'format' erase.
A free small piece of software called CCleaner has a 'Drive Wiper' in its 'Tools' pane. Under 'Security' it shows 'Simple Overwright (1pass) It is these options which securely erase the data for good, unrecoverable. '1 pass' option is good enough but 3/7/35 pass options are there depending on your paranoia of the lengths people will go to to try retrieving that erased data
http://download.cnet.com/CCleaner/
If there are multiple hard disks in that computer, erase the ones without the OS first of course