Answer: To return the computer to its blank-slate state, you'll need to do four things: Back it up (please!), erase the drive, reformat it, and reinstall the operating system.
If your computer is running Windows 7, you'll need a copy of the operating system, which you likely got on a USB drive or DVD when you bought the computer. If you don't have it, though, you're not out of luck. You can download a copy from Digital River (both for the 32-bit version; and the 64-bit, both legitimate ways to get a new copy of the OS that you paid for when you purchased the machine) and install it on a USB drive with at least 4 GB of space with the Windows 7 USB/DVD Download Tool. To activate the downloaded OS, use your Windows 7 Product Key—find it on a sticker on your computer or with the Magical Jelly Bean Keyfinder program.