clipped from: windowsitpro.com   

UPDATE: I haven't tested this yet, as I don't have Vista Upgrade media to test, but I'm told that Microsoft's internal documentation does explain how to clean install Vista using an Upgrade version. It appears to be more of a workaround than a true clean install, however. Here's what it says.

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.

Why not make a backup image of Windows XP with a program like Acronis True Image 10.0? Its Vista compatible. If you have to reinstall Vista just load the Windows XP image on your drive, it takes about 2 minutes, and then reinstall Vista over Windows XP.