How do you wipe out a computer hard drive?

Well, there are several ways:

Getting Rid of the Data Physically and Destructively
One, if you don't need the hard drive after the "wipe," is to hammer a nail or two through the unplugged drive, rendering it unreadable to all but the most dedicated data recovery team. If you have an extremely powerful magnet, you could run that over the drive a couple times, too.

Starting Over With a Hard Drive
If you are simply trying to start over with your hard drive, you can use Windows' built-in formatter to erase and reformat the drive. In "My Computer," right-click the drive, select format, use all of the default settings (save for the label of the drive), and hit "Format." This will not work if the hard drive you are trying to wipe is your only or main hard drive, though, and is not completely secure for those of you who are trying to purge data permanently and securely.

If it is your main hard drive, then you can simply reinstall Windows or any other operating system on your computer. This is done by popping in the recovery CD's or installation CD's that most likely came with your computer. Doing this will wipe all data and start you off anew, though the nature of the reinstallation means that this method is about as secure as the previous method (read: not very secure).

Wiping Sensitive Data
One last option, for those of you who want to wipe your drive securely and do not want to reinstall Windows, is to use a third-party program to erase your hard drive for you. The technique that these programs use is to write a large file to your hard drive over and over again to get rid of any residual stored data. These programs will probably cost money, and should really only be used by those who are either selling their current hard drive, and want sensitive data off, or those who really need to remove all traces of their files. To find a program capable of this, Google "Hard Drive Eraser" or "Secure Hard Drive Delete."
One could possibly do the same thing for free by deleting all of the sensitive files by reformatting, reinstalling the OS, or just deleting them in Explorer, and then writing large files to the hard drive until it fills up. This method would rewrite all of the hard drive, and render all of the data almost completely unreadable.

I hope this was helpful. Good Luck!