You might have read my previous post titled Upgrading to Vista with clean install? Forget it. Paul Thurrot at Windows IT Pro has a work around. Here are the steps he wrote:
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.
I don’t have Vista to test this, so use it at your own risk.
Thanks to Cybernet for the info.