Did you buy a copy of Windows Vista or a PC running Windows Vista after June 26, 2009?
If you bought a qualifying PC
The upgrade fulfillment method may vary by PC manufacturer. When you buy a qualifying PC from a PC manufacturer that has chosen to participate in the Windows 7 Upgrade Option Program, you'll generally need to fill in a form and follow a few steps to order your upgrade copy of Windows 7. Look for a form with redemption details in the packaging that came with your PC. If you bought a qualifying PC and can't find out how to get the upgrade, please check on the PC manufacturer's web site.
