If you had xp home installed, you can upgrade to premium. If you had xp professional installed you can't upgrade to premium.
http://www.microsoft.com/windows/products/wind owsvista/buyorupgrade/upgradepaths.mspx
However, there is a way to use an upgrade version to do a clean install of Vista.
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.)
Rest on next post