Here's how to do it:
1. Boot with the Windows Vista Upgrade DVD. 2. Click "Install Now." 3. Do not enter a Product Key When prompted. 4. When prompted, select the Vista product edition that you do have. 6. Install Vista normally. 7. Once the install is complete, restart the DVD-based Setup from within Windows Vista. Perform an in-place upgrade. 8. Enter your Product Key when prompted.