A Windows 7 clean install simply means an installation of Windows 7 on an unused partition on your hard drive. In most cases, though, a clean install of Windows 7 means to remove an existing operating system (Windows XP, Linux, Windows 7, Windows 8, ... doesn't matter) and to replace it with a fresh installation of Windows 7.
After serious Windows 7 problems or during a new Windows 7 installation with an older operating system installed that you'd like to replace, it's best to wipe your primary hard drive partition clean and reinstall Windows 7 from scratch - a procedure referred to as a "clean install" or sometimes as a "custom install".
This guide is broken into 3 parts for a total of 34 steps and will walk you through every part of the Windows 7 clean install process.
Backup & Locate Your Product Key
The most important thing to realize before performing a clean install of Windows 7 is that all of the information on the drive that your current operating system is installed on (probably your C: drive) will be destroyed during this process. That means that if there's anything you want to keep you should back it up to a disc or another drive prior to beginning this process.
You should also locate the Windows 7 product key, a 25-digit alphanumeric code unique to your copy of Windows 7. If you can't locate it, there is a fairly easy way to find the Windows 7 product key code from your existing Windows 7 installation, but this must be done before you reinstall Windows 7.
Note: If Windows originally came preinstalled on your computer (i.e. you did not install it yourself), your product key is probably located on a sticker attached to the side, back, or bottom of your computer's case. This is the product key you should use when installing Windows 7.
Start the Windows 7 Clean Install Process
When you're absolutely sure sure that everything from your computer that you want to keep is backed up, proceed to the next step. Keep in mind that once you delete all of the information from this drive (as we'll do in a future step), the action is not reversible!
Note: The steps and screen shots shown in these 34 steps refer specifically to Windows 7 Ultimate edition but will also serve perfectly well as a guide to reinstalling any Windows 7 edition you may have including Windows 7 Professional or Windows 7 Home Premium.
Performing a Clean Install of an OS Other Than Windows 7?
See How Do I Perform a Clean Installation of Windows? for specific instructions for your version of Windows.