1. Boot your computer form the Windows 7 Upgrade DVD. When you get to the setup screen, start a new setup of Windows 7.
2. When prompted to enter your product key, Do not enter it. Instead click on the Next button and continue with the setup. This will install Windows 7 as a 30-day trial.
3. When prompted select the edition of Windows 7 which you had purchased. So if you purchase Home Premium, select Home Premium. This is important, so do not pick a version that you did not purchase as it will affect later steps.
4. Once the setup has completed and you are back at your new Windows 7 desktop, run the setup program from the Windows 7 DVD again from within the new Windows 7 30-day trial.
5. Go through the setup, but this time when it asks for your product key, enter in the product key that came with your upgrade.
6. When asked whether to perform an Upgrade or a Custom (advanced) install, choose the Custom (advanced) option to perform a clean install of Windows 7. Yes, you are performing a second installation of Windows 7.
7. Once setup has completed for the second time you will be able to activate Windows 7 as normal. You are now left with a fresh clean install of Windows 7 and your original Windows 7 install has been renamed C:\Windows.old. You can now delete the Windows.old folder as it is not necessary to be on your computer.