If you upgrade your computer or your OS is totally corrupted, it is preferable to do a clean installation. Unfortunately, the Windows 7 upgrade discs only allow you to upgrade if you already have Vista or a lower version of Windows 7 installed. In the past, you could put your old media disc in and it would allow you to do a clean install, but that functionality has been taken out. So how can you do a clean install with Windows 7 upgrade media without reloading the old OS?
The simplest way I have found is through a simple registry entry change. There seems to be other methods floating around the net, but this one has always worked flawlessly for me. So lets get to it!
Do a clean install from your upgrade disc. In the initial loading, press the skip button when it asks you for your product key. After you finish the load, try to activate your Windows 7 online manually. It may surprise you and work, but you will likely get the message below:
Description: The software licensing service determined that this specified product key can only be used for upgrading not for clean installations.
To get around this, do the following:
1. Ensure that there are no Windows Updates pending that would require a system reboot to finish installing. If there are, then restart the computer to let them finish installing before proceeding. Note: if an update reboot fouls things up, just repeat the steps 2 through 4 below.
2. Go to this registry entry and change it from a 1 to a 0.
3. Open an elevated command prompt (a command prompt with administrator rights). Click on All Programs and Accessories, then right click on Command Prompt and click on Run as administrator.
4. In the elevated command prompt, type slmgr -rearm and press Enter. You will get a confirmation message. Hit OK and close the command prompt window.
5. Restart computer and activate Windows 7 manually with your product key.
That's it! If you make a mistake just try steps 1 - 5 again.
IMPORTANT NOTE! These instructions are not meant to get around Microsoft licensing. It is ONLY for fully registered users that have the original Microsoft OS and want to do a clean install.
Full Disclosure: These instructions were found on the excellent website Window's SevenForums. I could have just provided a link, but their write up had a lot of extra material (different methods and more detail) and I wanted to have an abbreviated version for the more experienced user. Feel free to follow this link to see the original: http://goo.gl/H0G4h