It belongs mostly in software, so I'll move it for you
Anyways, there's many ways we can wipe a PC. First, if you have a Windows CD, you can install your own operating system, and in the wizard, you'll have the option to format (wipe basically) the hard drive. In Windows Vista/7, you have to click the advanced option in order to format the drive.
The second way is to download a utility boot CD that has tools and diagnostics and you can use a tool to format the hard drive from that. Check out This