Be careful about wiping your hard-drive completely blank, if you do wipe it... you won't have an Operating System... so if you don't have the disk for it, you'll need to get one before you do so. Also depends on how old the hard-drive is and if it's really worth saving; the older your computer, the more I would suggest that you buy a new hard-drive and grab a copy of a fresh OS install disk. That way you can save yourself the hassle when your hard-drive won't work anymore (Viruses and such can cause platter issues). If you decide to go with a new hard-drive and OS, it'll be a lot faster than wiping, reformatting, and installing an OS ontop of all of that. Like I said, I'd buy a new hard-drive but you can do whatever you want
All you'd really have to do when you get a disk, go into the OS recovery counsol and delete the partition with the OS installed on it, then continue to install the new OS. OR if you chose to go the way of a new hard-drive: open up the side of the computer (with the computer off and unplugged), touch the Power Supply (To discharge static-charge), unplug your HD from power and the ribbon cable in the back, plug in your new HD, close the side of the computer, power on with OS CD in the tray, install OS, done.Hopefully this answers your question.Cheers.