Full or upgrade retail version?

Does the content of the Full retail version of, say, XP Pro contain anything above and beyond the Upgrade retail version? Or, is the Full version only necessary if you don't have an OS installed on your system. There is a pretty hefty price difference between the two and a really hefty price difference between the Ultimate Vista full and upgrade versions.

There's no difference between the content of the Full and Upgrade versions at least up to XP, and I can't see why it would be different for Vista), each will produce the same Windows installation. The full version will entitle you to install onto any single computer system as long as you own the software. So unlike OEM software, it can be deleted from a previous system and transferred to a new system when you upgrade if you wish.

I'm open to correction or clarification, but I believe the Upgrade version (which requires evidence of a qualifying previous OS, either already installed or installation media) "extends" the licence of the OS that is being upgraded. ie if you upgrade an OEM system, the upgraded system is considered OEM, so the licence would not transfer to an upgraded system. If a full OS is upgraded, the upgrade is a full licence, and could be transferred.

That's the main reason for the lower cost, you're not getting a full licence, you're extending the licence of the existing software.

Thanks, Platypus. :)