I think I will stay with XP for the time being.
You actually can do a clean install of Vista upgrade version, (not as easy as I believe it should be)
How to Clean Install Windows Vista with Upgrade Media.
Step 1: Install Windows Vista
Boot your PC with the Windows Vista Upgrade DVD. After the preliminary loading screen, click the Install Now button to trigger Vista Setup. In the next screen, you normally enter your product key. However, there's a little-known trick in Windows Vista Setup whereby you can simply skip this phase and use the install media (Upgrade or Full, any version) to perform a clean install of virtually any Vista product edition. What you do is leave the Product Key field blank, deselect the option titled "Automatically activate Windows when I'm online," and then click Next. Vista Setup will ask you whether you would like to enter your Product Key before continuing. Click No.
In the next Setup screen, you'll be presented with a list of the Windows Vista product editions you can install. This list may vary from locale to locale, but in the US, you'll see Vista Home Basic, Home Premium, Business, Ultimate, and some N editions. Choose the product edition you actually own. You'll be asked to verify that you've chosen the correct version. Do so to continue past the End User License Agreement (EULA) screen.
In the next screen, you select the type of install. Choose Custom (Advanced) instead of Upgrade. Next, you choose the partition to which to install Windows Vista. If you need to format the disk, select the Drive options (advanced) option to do so and then continue.
Now, Setup copies the Vista install image to your PC, expands it, and installs Windows. This phase of Setup should take about 15 to 20 minutes and trigger at least one reboot. When Vista is installed, you'll step through the penultimate phase of Setup in which you enter, in succession, your user name and password, computer name, and the date, time, and time zone. Then Setup runs its final task, a performance test that could take about 5 minutes. If everything goes well, and you're running fairly modern hardware, you should hit the Welcome screen and, after logging on, the new Vista desktop less than 30 minutes after you began this process.
Step 2: Upgrade
What you've installed is decidedly temporary. You've got 30 days during which you can run this non-activated version of Windows Vista. If you try to activate Windows now, it will fail, because you've performed a clean install of Vista and you only have an Upgrade product key.
What to do, what to do? If you read Microsoft's support note carefully, you will have seen that the Upgrade versions of Vista support upgrading from "a compliant version of Windows, such as Windows Vista, Microsoft Windows XP, or Microsoft Windows 2000." Well, you just installed Windows Vista, so why not just upgrade from this install? That's right: You're going to upgrade the non-activated clean install you just performed, which will provide you with a version of the OS that you can, in fact, activate.
To do this, just open Computer and double click on the icon for the DVD drive that contains the Vista Upgrade media. Run Setup again, this time from within Vista. Choose Install Now, and then "Do not get the latest updates for installation" in the next screen. Then, in the now-familiar Product Key phase, enter your product key. It's on the back of the pull-out Vista packaging. You can choose to automatically activate Windows when online or not, it's your choice. In the next screen, accept the Windows EULA.
Now, choose the Upgrade option. Windows will install as before, though you might notice that it takes quite a bit longer this time. (Upgrade installs seem to take up to 45 minutes, compared to 30 minutes or less with clean installs, and reboots at least one additional time.)
Because you've just completed an upgrade install, you won't be prompted to enter your user name and so forth (only the time zone screen is presented). Instead, you'll just boot directly to the Welcome screen when the performance check is complete. Using the user name and password you created during the first install, logon to Windows.
Once again, you have 30 days in which to activate Vista. However, this time activation will work: To activate Vista immediately (unless you told it to do so during Setup), open the Start Menu, right-click Computer, and choose Properties. Then, at the bottom of the System window that appears, click the link titled Activate Windows now.
Is this legal?
One might naturally wonder whether the aforementioned instructions describe an action that is legal or ethical. After all, anyone could purchase an Upgrade version of Windows Vista (therefore saving a lot of money when compared to a Full version) and use it to perform a clean install even if they don't own a previous, compliant Window version.
Fair enough. Of course, if you do use this workaround to clean install Vista with the Upgrade media, and you don't own a previous, compliant version of Windows, you're most certainly violating the Windows EULA and, thus, breaking the law. Proceed at your own risk.
Now wonder they do not make it known, or easy to clean install an upgrade Vista disk when they could make more money from selling people who don't like to do an upgrade install compared with a clean install the more expensive full version.