Found the following ... might be useful :)
Good old MS ;-)
The secret is that the setup program in Vista's upgrade version will accept an installed copy of XP, W2K, or an unactivated copy of Vista itself as evidence of a previous installation.
This enables you to "clean install" an upgrade version of Vista to any formatted or unformatted hard drive, which is usually the preferred method when installing any new operating system. You must, in essence, install Vista twice to take advantage of this trick. But Vista installs much faster than XP, so it's quicker than installing XP followed by Vista to get the upgrade price.
Before you install Vista on a machine that you don't know is 100% compatible, you should run Microsoft's free Upgrade Advisor. This program which operates only on 32-bit versions of XP and Vista (plus Vista Enterprise) reports to you on any hardware or software it finds that may be incompatible with Vista. See Microsoft's http://windowssecrets.com/links/mxw8ufp4ohind/983c85h/
Upgrade Advisor page .
Also, to see which flavors of XP Home, XP Pro, and 2000 officially support in-place installs and clean installs of the different Vista editions, see Microsoft's http://windowssecrets.com/links/mxw8ufp4ohind/e41f9bh/
upgrade paths page .
Here's a simplified overview of the steps that are required to clean-install the upgrade version of Vista:
Step 1. Boot the PC from the Vista DVD.
Step 2. Select "Install Now," but do not enter the Product Key from the Vista packaging. Leave the input box blank. Also, turn off the option Automatically activate Windows when I'm online. In the next dialog box that appears, confirm that you really do want to install Vista without entering a Product Key.
Step 3. Correctly indicate the version of Vista that you're installing: Home Basic, Home Premium, Business, or Ultimate.
Step 4. Select the "Custom (Advanced)" install, not the "Upgrade" install.
Step 5. Vista copies files at length and reboots itself one or more times. Wait for the install to complete. At this point, you might think that you could "activate" Vista, but you can't. That's because you haven't installed the Vista upgrade yet. To do that, run the DVD's setup.exe program again, but this time from the Vista desktop. The easiest way to start setup again is to eject and then reinsert the DVD.
Step 6. Click "Install Now." Select Do not get the latest updates for installation. (You can check for these updates later.)
Step 7. This time, do enter the Product Key from the Vista packaging. Once again, turn off the option Automatically activate Windows when I'm online.
Step 8. On this second install, make sure to select "Upgrade," not "Custom (Advanced)." You're not doing a clean install now, you're upgrading to Vista.
Step 9. Wait while Vista copies files and reboots itself. No user interaction is required. Do not boot from the DVD when asked if you'd like to do so. Instead, wait a few seconds and the setup process will continue on its way. Some DOS-like, character-mode menus will appear, but don't interact with them. After a few seconds, the correct choice will run for you automatically.
Step 10. After you click a button labeled Start in the Thank You dialog box, Vista's login screen will eventually appear. Enter the username and password that you selected during the first install. You're done upgrading to Vista.
Step 11. Within 30 days, you must "activate" your copy of Vista or it'll lose functionality. To activate Vista, click Show more details in the Welcome Center that automatically displays upon each boot-up, then click Activate Windows now. If you've dismissed the Welcome Center, access the correct dialog box by clicking Start, Control Panel, System & Maintenance, System. If you purchased a legitimate copy of Vista, it should quickly activate over the Internet. (You can instead activate by calling Microsoft on the phone, which avoids your PC exchanging information with Microsoft's server.)