I have an XP installation that I thought I'd try to upgrade. All versions of Vista are on the same DVD. I had previously installed Ultimate, and I thought I'd try premium to see the differences first hand. Turns out you can't upgrade XP to Premium from the DVD I had. You could only do a clean install. You could upgrade to Business, Ultimate and I think Enterprise. I restarted the install this time putting in the key for business. The installer has the option of going online to see if their are any updates to the installer. This is new. Turns out I had to uninstall the Bluetooth Stack for Windows by Toshiba. I'm going to restart the installation now.
Its upgrading right now, and the installation dialog says at the bottom:
"Your upgrade may take several hours to complete"!
Thankfully it didn't take several hours, just one, and the install was pretty normal after that.