From Microsoft:
http://emea.microsoftstore.com/UK/Mi...3_HPup_default
Running Windows Vista?
If you have Windows Vista, you can purchase Windows 7 Upgrade versions. You can do a clean install (back up your files, clean install, and reinstall your applications) or an in-place upgrade (Windows 7 installs over Windows Vista).
Running earlier versions?
If you have Windows XP, you can purchase Windows 7 Upgrade versions. But you must back up your files, clean install, and reinstall your applications.
If you’re running Windows 2000, you’ll need to purchase the full product and do a clean install.