Whether you're going to tear out your old drive and replace it; or wipe and reuse your old one, no computer will run without an operating system. If you have a PC that's running, say, Windows 7, it probably came with a backup disk that you can use to reinstall the operating system after you've done your cleanup.
If not, don't worry. You can snag another copy from Microsoft via the Digital River Web site, and install it on a USB drive with at least 4 gigabytes of space, using the Windows 7/USB/DVD download tool. To activate the software, you'll need the Windows 7 product key, which should be on a sticker on your computer case. If you can't find it, download a free program called Magical Jelly Bean Keyfinder that will locate the product key for you [source: Arndt].
If you've got a really old Mac, you'll need a disk with the operating system installation files as well. But if you've got a recent release such as Lion or Mountain Lion, you don't need one. Instead, after you erase the data on your drive, you can run a utility that will download the OS from Apple and reinstall it. We'll give you more detailed instructions later [source: Arndt].